UC-NRLF 


^D    bT    252 


THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

DAVIS 


1 


3"? 


APPLICATION 


SCIENCE    OF    MECHANICS 


PRACTICAL    PURPOSES. 


JAMES  RENWICK,  LL.D., 

ROFESSOR   OP    NATURAL   EXPERIMENTAL   PHILOSOPHY   AND 
CHEMISTRY    IN   COLUMBIA   COLLEGE. 


•     NEW    YORK: 
HARPER  &   BROTHERS,   PUBLISHERS, 

TBAITELIN  SQUARE. 
1860. 

LlBRARia 

DKIVBRSITY  OF  CALIFOKNBI 
DAVIS 


Entered,  according  to  Act  of  Congress,  in  the  year  1840,  by 

Harper   &   Brothers, 
In  the  Clerk's  Office  of  the  Southern  District  of  New  York. 


PREFACE. 


In  the  work  which  is  now  presented  to  the  public, 
the  author  has  endeavoured  to  exhibit  in  as  popu. 
lar,  and,  at  the  same  time,  as  condensed  a  form  as 
possible,  the  principles  and  leading  facts  of  the  ap- 
plication of  the  theory  of  mechanics  to  useful  pur- 
poses. With  this  view,  the  nature  and  mode  of  ac- 
tion of  the  prime  movers  which  are  employed  in  the 
arts,  and  the  engines  through  whose  intervention 
they  are  brought  into  efficient  action,  have  been 
briefly  considered ;  a  selection  of  useful  machines 
has  been  introduced,  as  an  illustration  of  the  appli- 
cation of  these  prime  movers  ;  and  the  machinery 
used  in  those  manufactures,  which  have  either  been 
successfully  introduced  into  the  United  States,  or 
promise  to  be  of  value  to  our  country,  have  been  ci- 
ted as  practical  instances  of  the  manner  in  which 
the  natural  agents  have  been  brought  to  the  aid  of 
human  industry. 

In  treating  of  these  subjects  it  has  been  attempted 
to  be  as  brief  as  is  consistent  with  an  intelligible  ex- 
planation of  them.  If,  then,  it  is  hoped  that  the 
work  may  not  be  without  its  value  to  practical  men, 
it  is  not  intended  to  supersede  more  extensive  trea- 


CONTENTS. 


I.   INTRODUCTION, 

flection  Pase 

1.  Definition  of  Machines 13 

2.  Reasons  for  the  use  of  Machines 14 

3.  Division  of  Machines 16 

4.  Different  kinds  of  motion  in  Machines,  and  their  combi- 

nations            ib. 

5.  Combinations  of  motion  found  in  Machines     .        .        .  17 

6.  Points  in  Machines  whose  motion  is  most  important      ,  22 

7.  Prime  Movers  used  in  Practical  Mechanics    .        .        .  ib. 

8.  Impossibility  of  Perpetual  Motion    .        .        .        .        .24 

9.  Measure  of  the  action  of  a  Prime  Mover         .        .        .  ib. 

10.  Dynamical  equilibrium  of  Machines         .        .        .        .  ib. 

11.  Most  advantageous  velocity  of  the  impelled  point  of 

Machines 25 

12.  Variations  in  the  motion  of  Machines      .        .        .        .  ib. 

13.  Principle  on  which  the  Fly-wheel  acts   ....  26 

14.  Other  applications  of  that  principle  .        .        .        .27 

15.  Principle  and  description  of  the  Governor       •        •        ,  ib. 

II.   OF   PRIME    MOVERS     .  •  .  .29 

1.  Of  Weights         .        .        .        .  ib. 

16.  Mode  in  which  a  Weight  is  applied  as  a  Prime  Mover  .  ib. 

2.  Of  Springs 30 

17.  Mode  in  which  a  Spring  is  applied  as  a  Prime  Mover,  and 

examples  of  their  use ib. 

3.    Of  the  Strength  of  Men  and  Animals       .         .  33 

18.  Animals  may  be  considered  as  Machines        .        .        .  ib. 

19.  Structure  of  Animals ib. 

20.  Mode  in  which  the  Bones  are  moved      .        .        .        .  ib, 

21.  Explanation  of  the  erect  posture  of  Man         .        .        .35 

22.  Relative  lengths  of  the  flexor  and  extensor  muscles  in 

Quadrupeds ib. 

23.  Relative  lengths  of  the  flexor  and  extensor  muscles  in 

Birds 36 

24.  Progressive  motion  of  Animals ib. 

25.  Walking  and  running  of  Men 37 

26.  Walking  and  running  of  Horses      .        ,        ,        ,        .  ib. 

27.  Flymg 38 

28.  Motions  of  Fishe.s 39 


VIU  CONTENTS. 

Section  Pj^ 

29.  Mode  of  estimating  the  force  of  Animals         .        .        .41 

30.  Comparison  and  estimates  of  the  strength  of  Men  and 

Animals  applied  to  draught ib. 

31.  Comparison  of  the  same  strengths  in  other  cases    .        .  43 

4.   Of  Water 46 

32.  Modes  in  which  a  circular  motion  may  be  produced  by- 

Water  ib. 

33.  Description  of  an  Undershot  wheel         .        .        .        .47 

34.  Maximum  effect  of  an  Undershot  wheel          .        .        .  ib. 

35.  Proper  position  of  its  buckets 48 

36.  Mode  of  increasing  its  power ib. 

37.  Uniiershot  wheel  of  Poncelet,  and  rules  for  estimating 

the  force  of  Undershot  wheels ib. 

38.  Overshot  wheel 50 

39.  Velocity  with  which  the  water  should  fall  on  an  Over- 

shot wheel ib 

40.  At  what  point  the  water  should  be  introduced  01 

Overshot-wheel 55^ 

41.  Measure  of  the  force  of  an  Overshot  wheel    .        .        .  b'a 

42.  43.  Modes  of  constructing  the  buckets  of  an  Overshot 

wheel ib. 

44.  Description  and  use  of  a  Breast  wheel    .        .        .        .54 

45.  Reacting  wheel,  or  Barker's  Mill 55 

46.  Improvement  on  Barker's  Mill 67 

47.  Wheel  reacting  beneath  the  surface  of  Water        .        .  ib. 

48.  Spiral  Reacting  wheel ib. 

49.  Limit  to  the  use  of  an  Overshot  wheel,  and  substitute 

when  the  limit  is  reached 59 

50.  Horizontal  wheels  by  impulse ib. 

51.  Horizontal  wheels  with  spiral  channels  .        .        .        .  60 

62.  Danaide ib. 

5.  Of  the  Wind         ....  61 

63,  54,  55,  56.  Windmills ib. 

6.  Of  Steam 64 

57.  Generation  and  tension  of  Steam ib, 

58.  Relation  between  the  tension  and  volume  of  Steam        .  ib. 

59.  Boilers ib.  i 

60.  Causes  of  the  decay  of  Boilers 65  i 

61.  Materials  and  strength  of  Boilers 66 1 

62.  Figure  of  Boilers 67 ! 

63.  Length  of  the  flues  of  Boilers  ......  70 

64.  Quantity  of  Steam  generated  by  Boilers         .        ,        .  ib. 

65.  Dimensions  of  the  Furnaces  of  Steam-engines       .        ,  ib. 

66.  Dangers  arising  from  defect  of  Water     .        *        .        .71 

67.  Gauge-cocks  and  water-gauge 72 

68.  Feeding  apparatus 73 

69.  Steam-gauge 74 


CONTENTS.  IX 

Section  l^ge 

70.  Safety-valve 74 

71.  Use  of  a  Thermometer 75 

72.  Valves  of  fusible  metal ib. 

73.  Dampers 76 

74.  Precautions  to  be  observed  in  the  use  of  Boilers    .       .  ib. 

75.  Proof  of  Boilers       .        .        .        ,        .        .        .        .  ib. 

76.  Steam-engines ib. 

77.  Savary's  Engine 77 

78.  Objections  to  Savary's  Engine 80 

79.  Newcomen  and  Cawley's  Engine ib. 

80.  Defects  in  Newcomen  and  Cawley's  Engine  .        .        .82 

81.  Improvement  discovered  by  Watt 83 

82.  Hot-water  pump 84 

83.  Cold-water  pump ib. 

84.  Steam  used  by  Watt  as  the  moving  power     .        .        .  ib. 

85.  Plug-frame  and  Hand-gear       ......  85 

-  *'6.  Description  of  Watt's  single-acting  Engine    .        .        .  ib. 

r."  Ascription  of  Watt's  double-acting  Engine  .        .        .  ib. 

88.  Throttle  valve 89 

89.  Steam  chests  and  side  pipes ib. 

90,91.  Puppet  valves ib. 

92.  Side  valves      .        .        .        .     '  .     '   .        .        .        .90 

93.  Description  of  the  Cylinder 91 

94.  Piston  and  its  packing ib. 

95.  Description  of  the  Condenser 93 

96.  Foot  valve ib. 

97.  Air-pump ,  ib. 

98.  Cold-water  Cistern 94 

99.  100.  Cold  and  Hot  water  Pumps ib. 

101.  Relative  dimensions  of  Cylinder,  Condenser,  and  Air- 

pump     ib. 

102.  Vacuum-gauge ib. 

103.  Changes  necessary  in  the  engines  of  Steamboats   .        .  ib. 

104.  105,  106.  Working-beam  and  Parallel  motion          .        .  95 

107.  Crank 98 

108.  Eccentric .        .        .  ib. 

J9.  Modes  of  estimating  the  power  of  Steam-engines  .        .  99 

10,  HI,  112.  Condensing  Engines  acting  expensively  .        .  101 

113.  Mode  of  using  high  steam 103 

114.  Difference  in  structure  of  high  pressure  and  condensing 

Engines ib. 

115.  Valves  of  high  pressure  engines ib. 

116.  Horizontal  Engines 104 

1 17, 1 18.  Comparison  of  condensing  and  high  pressure  Engines  ib. 

119.  Avery's  Engine ib. 

120.  Rotary  Engines 105 

III.   MACHINES   MOVED   BY   DESCENDING  WEIGHTS       .  106 

121.  Mode  of  regulating  the  descent  of  a  Weight   .        .        ,  ib. 

122.  Use  of  the  Pendulum  as  a  regulator       ...  ib. 

2 


^^; 


X  CONTENTS 

Section  Fa^ 

123.  Composition  and  object  of  a  Clock 107 

124.  Relation  between  the  weight  and  the  loss  of  motion  in 

the  Pendulum ib. 

125.  Compensation  Pendulums 108 

126.  Barrel,  Ratchet,  and  Ratchet-wheel       .        .        .        .109 

127.  Description  of  the  common  Clock ib. 

128.  Objections  to  the  Crown- wheel  and  Pallets     .        .        .113 

129.  Clocks  by  Franklin,  Ferguson,  and  Breguet  .        ,        .    ib. 

130.  Different  kinds  of  Scapements 115 

131.  Astronomic  Clocks 117 

132.  Division  of  labour  in  the  construction  of  Clocks     .        .  118 

IV.   MACHINES    MOVED   BY    SPRINGS      .  .  .    119 

133.  Case  in  which  Springs  are  most  frequently  used    .        .    ib. 

134.  Principle  of  the  Fusee ;  description  of  Barrel,  Fusee,  and 

Chain ib. 

135.  Ratchet  and  Maintaining  Spring 120 

136.  Number  of  wheels,  &c.,  in  the  common  Watch     .        .  ib. 

137.  Regulator ib. 

138.  Compensation  Curbs  and  Balances          ....  121 

139.  Different  kinds  of  Scapements 122 

140.  Principle  on  which  the  Fusee  and  Chain  may  be  dis- 

pensed with 125 

141.  Chronometers ib. 

142.  Description  of  the  common  Watch          ....  126 

143.  Comparison  of  Watches  and  Clocks        ....  127 

144.  Division  of  labour  in  Watch-making        .        .        .        .  ib. 

V.    MACHINES   MOVED   BY   MEN   AND   ANIMALS    .  .    128 

145.  Other    Prime   Movers   more   advantageous    than   the 

strength  of  Man ib. 

146.  Description  of  the  Crane ib. 

147.  Description  of  the  Gin  or  Triangle 130 

148.  Description  of  the  Derrick ib. 

149.  Description  of  the  Pile  Engine 132 

VI.   OF   WHEEL   CARRIAGES   AND   ROADS  •  .   136 

150.  Measure  of  the  force  of  a  Horse  indraught     .        .        .    ib.    ' 

151.  Principles  on  which  wheels  are  applied  .        .        .        .    ib. 

152.  Proper  diameter  of  Wheels 137 

153.  154,  155.  Comparison  of  two  and  four  wheeled  carriages  138 

156.  Relative  heights  of  the  fore  and  hind  Wheels  .        .  141 

157.  Mode  of  compensating  a  difference  in  the  strength  of 

Horses 142 

158.  Structure  of  the  Wheels  of  Carriages 

159.  Advantages  of  broad  Wheels   . 

160.  Value  of  Springs  applied  to  carriages 

161.  Materials  proper  for  Road-making  . 

162.  Breadth  of  the  carriage-way  of  Roads 
163, 164.  Cross  section  of  Roads     . 


CONTENTS.  XI 

Section  ftg» 

165.  Ditches  and  Culverts 149 

166.  Gravel  as  a  material  for  Roads ib. 

167.  Principles  on  which  the  Slope  of  Roads  depends  .        .  150 

168.  Rules  for  laying  out  Roads 152 

169.  Pavements  of  Stone 154 

170.  Wooden  Pavements 156 

171.  Pavements  of  Asphaltum 157 

VII.    RAILROADS  .  .  .  .158 

172.  Origin  and  progress  of  Railroads ib. 

173.  Materials  and  construction  of  Railroads  .        .        .        .159 

174.  Grade  of  Railroads .161 

175.  Comparison  of  common  and  Rail-roads,  when  horses  are 

used ib. 

176.  Advantages  in  the  use  of  Steam  on  Railroads         .        .  162 
177, 178.  Principles  on  which  Steam  is  applied  to  move  car- 
riages on  Railroads ib. 

179,  180.  Description  of  Locomotive  Engines         .        .        .164 

181.  Effect  of  inclination  in  the  road  on  the  action  of  Loco- 

motive Engines 166 

182.  Self-acting  incHned  planes  on  Railroads         .        .        .  167 

183.  Effects  of  curves  in  Railroads ib. 

184.  Breadth  of  the  track  of  Railroads 168 

185.  Performance  of  Locomotive  Engines      ....  169 

VIII.    CANALS  AND   DOCKS  ,  .  .    171 

186.  Cases  in  which  na'^^gable  canals  are  used       .        .        .    ib. 

187.  Feeders  and  Reservoirs ib. 

188.  Estimate  of  water  intercepted  by  a  Feeder     .        .        .  172 

189.  Principles  on  which  the  dimensions  of  Canals  are  deter- 

mined    ib. 

190.  Cross  section  of  Canals ib. 

191.  Canal  Locks 173 

192.  Proper  height  of  Locks ,  176 

193.  Inclined  Planes  for  Canals 177 

194.  Estimate  of  Water  for  the  supply  of  Canals    .        .        .    ib. 

195.  Waste  Gates,  Wiers,  Culverts,  and  Aqueducts       .        .  179 

196.  Wet  Docks ib. 

197.  Dry  Docks 180 

198.  Aqueducts  for  the  supply  of  cities 181 

199.  Natural  process  in  which  water  is  purified     .        .        .    ib. 

200.  Value  of  open  channels  for  the  supply  of  cities       .        .182 

201.  Use  of  Reservoirs 183 

202.  203.  Use  of  Water-wheels  and   Steam-enghies  for  the 

supply  of  cities  with  Water 184 

204.  Modes  of  crossing  valleys  with  Aqueducts      .        .        .    ib. 

205.  Modes  of  supplying  Water  at  different  levels  .        .186 

206.  Mode  of  distributing  Water  in  pipes        .        .        .        .    ib. 

207.  Obstructions  to  which  pipes  are  subject,  and  modes  of  re- 

moving them         187 


XU  CONTENTS. 

Section  Feu|^ 

IX.    HYDRAULIC   ENGINES  .  .  .188 

1.  Fountain  of  Hero      .         •         ,         ,    ib. 

208.  Description  of  the  Fountain  of  Hero        .        ,        .        .    ib 

2.  Machine  of  Schemnitz  ....  190 

209.  Description  of  the  Machine  of  Schemnitz       .        .        .    ib. 

3.  Pump  of  Vialon      .        .        .        .191 

210.  Description  of  the  Pump  of  Vialon         .        ,        •        .    ib. 

4.  Bucket  Machine      ....  192 

211.  Machine  composed  of  single  Buckets      .        ,        ,        .    ib. 

212.  Machine  composed  of  chains  of  Buckets         .        .        .  193 

5.  Siphon  of  Venturi    .        ,        ,        .195 

213.  Description  of  the  Siphon  of  Venturi      .        .        .        .    ib. 

6.  Hydraulic  Ram        ....  195 

214.  Principles  and  description  of  the  Hydraulic  Ram  of 

Mongolfier ib. 

7.  Pumps 198 

215.  Different  valves  used  in  Pumps ib. 

216.  Value  of  the  common  Pump 203 

217.  Pumps  without  friction ib. 

218.  Eflfect  of  atmospheric  pressure  in  the  common  Pump    .  204 

219.  220.  Forcing-pumps 205 

221.  Fire-engine 208 

222.  Rotary  Pumps 209 

8.  Pump  of  Vera        •        •        .        ,  210 

223.  Description  of  the  Pump  of  Vera     .       ,       ,       ,        ,    ib. 

9.  Centrifugal  Pump      ,        ,        ,        ,211 

224.  Description  of  the  Centrifugal  Pump      .       ,        .        ..   ib. 

10.  Chain  Pump  .        ,        ,        .212 

225.  Vertical  Chain  Pump      .......    ib. 

226.  Inclined  Chain  Pump       .        .        .        ,        ,        .        .214 

227.  Dredging  Machine ib. 

11.   Screw  of  Archimedes     .        .        ,         .218 

228.  Principle  and  original  form  of  the  Screw  of  Archimedes    ib. 

229.  Other  forms  of  that  Instrument ib. 

12.  Flash  Wheel 217 

230.  Description  and  performance  of  the  Flash  Wheel  .        .    ib. 

231.  Other  Hydraulic  Engines 219 

13.  Hydraulic  Press       .        .        ,         .    ib. 

232.  Principle  and  structure  of  the  Hydraulic  Press       .        .    ib. 

233.  Applications  of  the  Hydraulic  Press        .        .        .         .219 


CONTENTS.  Xlll 

BecUoD  Page 

X.    EQUILIBRIUM  AND   MOTION  OP   VESSELS   .  .  222 

234.  Principles  of  the  equilibrium  of  Vessels  .        .        .        .    ib. 

235.  Tendency  of  Ships  to  a  change  of  figure  .        .        .223 

236.  Modes  of  preventing  or  lessening  this  Change        .        .  225 

237.  Rolling  of  Vessels 227 

238.  Modes  of  lessening  the  violence  of  Rolling ;  Lifeboat  of 

Greathead 229 

239.  Applications  to  Vessels  with  Saiis  and  to  Steamboats    .  231 

240.  Pitching  of  Vessels,  and  modes  of  lessening  its  violence  233 

241.  Resistances  which  oppose  the  progressive  motion  of  Ves- 

sels ;  Water-lines  of  Vessels 234 

242.  Modes  in  which  Vessels  are  propelled     ....  236 

243.  Principles  of  the  action  of  the  Wind  upon  Sails     .        .    ib. 

244.  Beating  and  plying  to  windward 237 

245.  Principles  of  the  action  of  the  Rudder    ....  241 

246.  Tacking :        ....  243 

247.  Veering  or  wearing         .        .        .        .        .        .        .  244 

248.  Best  mod^  of  increasing  the  area  of  Sails       .        .        .    ib. 

249.  Position  of  the  Masts  of  Vessels 245 

250.  Application  of  Steam  to  Vessels;  Paddle-wheels  .        .    ib. 

251.  252.  Consideration  of  the  theory  of  the  action  of  Paddle- 

wheels  246 

253.  Figure  and  velocity  of  Steamboats  ....  250 

254.  Modifications  required  to  adapt  the  Steam-engine  to  Nav- 

igation   ib. 

255.  Use  of  Steam  upon  Canals 251 

XI.  MACHINES  USED  IN  MANUFACTURES. 

256.  Agents  employed  in  driving  manufacturing  Machines    .  253 

257.  Necessity  for  changing  the  motion  of  the  engine  on 

which  the  Prime  Mover  acts ib. 

258.  First  mode  of  combining  Wheels  and  Axles    .        .        .254 
359.  Second ib. 

260.  Third 256 

261.  Wheel  and  Pinion ;  modes  of  changing  the  plane  in  which 

the  motion  of  Wheels  and  Pinions  are  performed        .  257 

262.  263.  Principles  on  which  the  action  of  Wheels  and  Pin- 

ions rest 259 

Notes  to  261  and  263.  Method  of  drawing  the  figures  of  the 
teeth  of  wheels ;  illustrations  of  the  mode  of  combi- 
ning wheels  and  pinions,  drawn  from  planetary  ma- 
chines and  the  machine  for  proving  chain  cables         .  260 

Flouring  Mills 262 

264.  Importance  of  Flouring  Mills  in  the  United  States  .    ib. 

265.  Description  of  Millstones  and  their  accessories       .  .  263 

266.  Dimensions  and  product  of  Millstones      .        .        .  .264 

267.  Labour-saving  apparatus  in  Flouring  Mills      .        .  .  265 

268.  Prime  Movers  applicable  to  Flouring  Mills     .        .  .  268 

269.  Rules  and  Tables  for  Flouring  Mills       .        .       .  .  269 


XIV  CONTENTS. 

Section  Fags 

Saw  Mills 271 

270.  Description  of  Saw  Mills         ....  .    ib. 

271.  Circular  Saws 273 

272.  Planing,  with  Tonguing  and  Grooving  Machines    .        .    ib. 

Cotton  Spinning         ....  274 

273.  Modes  of  separating  the   seed  of  Cotton;   Whitney's 

Saw-gin ib. 

274.  Blowing  and  Batting  Machines 275 

275.  Carding  Machines 277 

276.  Original  methods  of  Spinning 280 

277.  Great  Spining-wheel ib. 

278.  Small  Spinning-wheel 281 

279.  Improvement  m  the  art  of  Spinning  by  the  application 

of  Steam  and  Water  power 283 

280.  Drawmg  Machine  * 284 

2S1.  Roving  Machine 286 

282.  Two  kinds  of  Spinning     .        .        .        .        .        .        .287 

283, 284,  285.  Mule  Spinning ;  Double  Speeder  and  Stretching 

Frame 287 

286.  Throstle  Spinning 288 

287.  Extent  to  which  the  process  of  Drawing  has  been  carried  291 

288.  Rate  of  the  motion  of  Mules ib. 

289.  Self-acting  Mule ib. 

290.  Efifect  of  Cotton  Manufacture  on  comfort  and  morals     .  292 

Flax  Spinning  .....  293 

291.  292,  293.  Processes  of  Flax  Spinning      .        .        .        .293 

Spinning  of  Woollen  and  Worsted  .         •         .  295 

294.  First  process  of  the  Woollen  Manufacture      .        .        .    ib. 

295.  Scribbling ib. 

296.  Slubbing,  &c 296 

297.  298.  Combing ib. 

299.  Breaking 298 

Silk  Manufacture 298 

300.  Silkworm  in  the  United  States        ,        ,        .        .        .    ib. 

301.  Reeling ib. 

302.  Raw  Silk 299 

303.  Winding 300 

304.  Doubling  and  Throwing ib. 

Weaving  and  Finishing  •         •        •         •  ib. 

305.  Principles  of  Weaving     .......  ib. 

306.  Common  loom ib. 

307.  Shuttle ....  303 

308.  Tweel  and  Patterns ib. 

309.  Advantage  of  the  Fly-shuttle                    •        .        .        *  p 

310.  Power  Loom    ...                       ,        .       ,        .  ib. 


CONTENTS. 


Seetion 

311.  Calendering 

312.  Fulling      . 

313.  Milling     . 

314.  Dressing  . 

315.  Shearing  . 


316. 
317. 


318. 
319. 
320. 
321, 
324. 
325. 
326. 
327. 
328. 
329. 
330. 
331, 
333. 
334. 
335. 
336. 
327. 


Printing  Machines 
History  of  Printing  Press 
Applegartlrs  Cowper's  Press  , 


XII.    MINING 


Definition  of  Mines 
Minerals  sought  in  Mines 
Characters  of  Mineral  Veins    . 
322,  323.  Modes  of  seeking  for  Mines 
Importance  of  surveys  in  Mining     . 
Galleries  and  Shafts 

Boring 

Mines  open  to  the  day 

Different  positions  in  which  valuable  Minerals 

Preparatory  works   .... 

Works  of  Research 

332.  Modes  of  extracting  Ore  . 

Dangers  of  Mining  .... 

Modes  of  supporting  Shafts  and  Galleries 

Drainage  of  Mines  .... 

Ventilation  of  Mines 

Conclusion      .       .  .       . 


XV 

Page 
303 

305 
ib. 
ib. 
ib. 

306 

,    ib. 

ib. 


310 
ib. 
ib. 
ib. 


.  311 

.  314 

.  315 

.  ib. 

.  ib. 

J  found  316 

.  ib. 


318 

ib. 


,  321 

,    ib. 

,  322 

,  323 

327 


OUTLINES 

or 

PRACTICAL   MECHANICS. 


INTRODUCTION. 

1.  Machines  are  defined  by  writers  on  the  theory 
of  Mechanics  as  instruments  by  which  the  direction 
or  intensity  of  a  force  is  changed.  In  elementary 
treatises  they  are,  in  consequence,  considered  as  sys- 
tems of  forces,  and  in  all  the  variety  of  their  forms 
may  be  reduced  to  a  single  principle,  known  as  that  of 
virtual  velocities.*  By  this  it  would  appear,  that  in 
no  machine  is  there  any  actual  gain  ;  for,  whenever 
the  power  or  intensity  of  a  force  is  increased  by  it,  the 
space  described  in  a  given  time  is  as  much  dimin- 
ished ;  and,  when  velocity  is  gained,  the  intensity 
with  which  the  moving  force  is  diminished  is  in  a 
like  proportion. 

In  proceeding  a  step  farther,  we  find  that  in  every 
machine  there  is  a  positive  loss  of  the  force  applied 
to  work  it,  for  its  motion  is  opposed  by  various  re- 
sistances, among  which  friction  is  the  most  important. 

When  we  consider  machines  in  their  practical  ap- 
plication, we  find  them  interposed,  like  tools,  between 
some  natural  agent  or  worker,  and  a  work  to  be  per- 

*  See  Mosely's  Illustrations  of  Mechanics.  Renwick's  Ele- 
ments of  Mechanics,  book  ii.,  chap.  vii. 


14  PRACTICAL  MECHANICS. 

formed,  in  order  to  render  that  work  capable  of  beina 
executed  which  would  have  been  difficult  or  evef 
impossible  without  the  aid  of  some  instrument. 

2.    Machines  are  interposed  between  a   natural 
agent  and  the  work  to  be  performed  for  severa!  rea 

r/iit?' '''  '°'-'^"^  "^  ^'-  --  frequ:^ 

(1.)  To  accommodate  the  direction  which  the  mo 
ving  power  necessarily  has,  or  that  in  which  it  is 
capable  of  acting  to  the  greatest  advantage,  to  the 
direction  in  which  alone  the  resistance  can  be  over! 
TZth.  h  '  ^^"^  ^  T^"  '^^^  "  ^'''ght  to  raise.to 
a  fixli  ;S,  '"^^'/"f'^^d  of  carrying  it  upward,  place 
a  fixed  pulley  a  little  above  the  point  to  which  the 

fasten  the  weight  to  it.  He  may  then  take  hold  of 
the  opposite  end  of  the  rope,  and,  pulling  downward 
upon  It,  draw  the  weight  upward. 

(2.)  A  natural  agent  may  have  a  fixed  and  deter, 
minate  velocity,  or  it  may  be  capable  of  working  to 
the  greatest  advantage  at  some  given  velocity,  while 

othei  velocity.  A  machine  may  therefore  be  ad- 
van  ageously  applied,  in  this  case,  to  change  the  ve- 
locity of  the  agent  into  that  best  suited  for  the  per. 
formance  of  the  work.  Thus,  as  we  shall  hereafter 
see,  water  falling  through  some  given  height,  gives 
It  wilT  H  *t  '^'"^'"  determinate  velocity,  at  which 
It  will  do  the  greatest  possible  quantity  of  work. 
Such  a  wheel  may  be  applied  to  drive  the  millstones 
used  in  grinding  grain,  and  these  must  have  a  par- 
ticular  speed,  m  order  to  do  the  work  efiectually. 
I  he  intervention  of  a  machine,  in  this  case,  is  not 
only  convenient,  but  absolutely  necessary, 

(o.)  A  natural  agent  may  be  capable  of  exertina 
no  more  than  a  certain  degree  offeree,  while  it  may 


PRACTICAL    MECHANICS.  15 

be  required  to  overcome  a  resistance,  or  remove  an 
obstacle  requiring  the  exertion  of  a  greater  force. 
A  machine  may,  in  this  case,  enable  the  natural 
agent  to  overcome  this  more  intense  resistance,  or 
to  remove  an  obstacle  which  it  could  not  otherwise 
stir.  Thus  a  single  man  may  wish  to  lift  a  stone  or 
other  weight,  so  great  that  it  cannot  be  moved  by  his 
unassisted  strength.  In  this  case,  by  laying  a  prop 
on  the  ground,  on  which  a  strong  bar  of  wood  or  iron 
is  laid,  he  constructs  a  machine  known  under  the 
name  of  the  lever.*  By  the  aid  of  this  he  can  move 
a  weight  which  would,  without  this  aid,  have  required 
the  united  strength  of  several  men.  But  the  rate 
at  which  the  weight  is  Ufted  is  as  much  less  than 
that  with  which  he  could  lift  a  stone  of  no  greater 
weight  than  he  could  easily  have  moved,  as  the 
weight  of  the  latter  is  less  than  that  of  the  former. 
In  this  case  a  work  is  performed  which,  to  a  single 
man,  would  have  been  impossible  without  the  inter- 
vention of  a  machine  ;  or  a  single  man  is  rendered 
capable  of  performing  what,  without  the  machine, 
would  have  demanded  the  united  strength  of  several 
men. 

A  farther  advantage  is  gained  by  the  use  of  ma- 
chines. Without  them  man  is  capable  of  using  no 
other  moving  force  than  his  own  strength,  or  that  of 
animals  used  only  as  beasts  of  burden  ;  but  when 
machines  are  employed,  he  becomes  capable  of  call- 
ing to  his  aid,  and  employing  in  the  execution  of  his 
duties,  a  number  of  other  natural  agents.  The 
most  useful  of  these  are  the  force  of  wind,  of  water, 
and  of  steam.  In  the  use  of  these,  the  duty  of  the 
men  who  have  charge  of  them  may  become  that  of 
superitendence  alone. 

3.  The  machines  which  are  used  in  practical  me- 

*  See  Mosely  (Harpers'  ed.).  ^  130. 


16  PRACTICAL    MECHANICS. 

chanics  may  be  either  simple  or  compound.  •  The 
simple  machines  are  only  six  in  number,  viz.,  the  Lev- 
er, the  Pulley,  the  Wheel  and  Axle,  the  Inclined  Plane, 
the  Wedge,  and  the  Screw.*  Compound  machines 
are  made  up  of  the  mechanic  powers,  combined  with 
each  other  in  various  ways,  and  modified  in  various 
manners.  In  these  combinations  there  is  not  only  a 
change  effected  in  the  direction  and  in  the  intensity 
of  the  moving  forces,  but  the  character  of  the  mo- 
tion  may  be  changed  also. 

4.  Of  the  lines  which  any  point  of  a  machine  can 
describe,  the  simplest  are  the  straight  line  and  the 
circle.  If  the  point  continue  to  move  forward  in 
the  same  line,  the  motion  is  said  to  be  continuous  and 
rectilineaL  Of  this  we  have  no  instance  in  the  parts 
of  machines  themselves,  but  it  is  often  found  in  prime 
movers. 

If  the  point,  after  having  described  a  straight  line, 
return  along  that  line  to  the  place  whence  it  first  set 
out,  the  motion  is  alternating,  and  is  said  to  be  recijp- 
rocating  rectilineaL 

If  the  point  describe  an  entire  circle,  turning  con- 
tinually in  the  same  direction,  the  motion  is  said  to 
be  continuous  circular. 

If  the  point  move  through  an  arc  or  portion  of  a 
circle,  and  return  along  that  arc  to  the  place  of  be- 
ginning, the  alternating  motion  is  said  to  be  recipro- 
cating circular. 

There  may  be  motions  either  continuous  or  recip- 
rocating, in  curves  other  than  a  circle.  It  is  not, 
however,  necessary  that  we  should  distinguish  these 
as  a  separate  class,  and  they  may,  in  most  cases,  be 
considered  as  performed  in  circles  whose  centre  is 
not  the  same  as  the  fixed  point  around  which  the  mo- 
tion is  performed.     Motions  of  this    character  are 

*  Renwick's  Elements  of  Mechanics,  book  iii.,  chap.  vi. 


PRACTICAL    MECHANICS.  17 

distinguished  by  the  name  of  eccentric^  and  when 
continuous  they  are  called  rotatory,  or,  in  its  more 
usual,  but  less  correct  form,  rotary, 

5.  Among  these  four  kinds  of  motions,  taken  by 
pairs,  ten  possible  combinations  exist ;  but  two  of 
these  never  occur  in  practice.  Machines  have  there- 
fore been  divided  into  eight  series,  viz.: 

(1.)  Those  in  which  a  continuous  rectilineal  mo- 
tion is  converted  into  another  of  the  same  description, 
Jbut  different  in  direction. 

Instance  a  simple  fixed  pulley,  Fig.  1.  In  this  a 
Fig.  1. 


weight,  attached  to  one  end  of  the  rope,  is  drawn  up- 
ward in  a  straight  line  by  a  force  which  draws  the 
opposite  end  of  the  rope  downward. 

(2.)  Those  in  which  a  continuous  rectilineal  mo- 
tion may  be  converted  into  one  continuous  and  circu- 
lar,  or  in  which  a  continuous  circular  is  converted 
into  a  continuous  rectilineal  motion. 

Thus,  in  the  water-wheel,  Fig.  2,  the  stream,  run- 
ning along  a  straight  channel,  strikes  against  the 
paddles,  and  gives  the  wheel  a  constant  motion  in 


18  PRACTICAL    MECHANICS. 

Fig.  2. 


the  same  direction ;  and  in  the  well-digger's  wind- 
lass, Fig.  3,  the  force  of  men  applied   to   handles 
Fig.  3. 


turns  the  axle  of  the  windlass,  and  the  hands  of  the 
men  describe  a  circle,  while  the  weight  fastened  to 
the  rope  is  drawn  upward  in  a  straight  line. 

(3.)  Those  in  which  a  continuous  rectilineal  mo- 
tion  is  converted  into  a  reciprocating  circular  mo- 


PRACTICAL    MECHANICS.  19 

tion.  For  the  illustration  of  this  case  we  shall  cite 
the  method  used  in  crossing  rivers,  known  under  the 
name  of  the  flying-bridge. 

Fig.  4. 


In  this  a  rope  is  fastened  at  some  distance  above 
the  point  where  the  crossing  is  to  be  effected,  and  is 
attached  to  the  ferry-boat  at  some  distance  from  its 
stem.  The  rope  being  over  a  support  on  that  side 
of  the  boat  which  is  opposed  to  the  current,  the  joint 
effect  of  the  tension  of  the  rope  and  the  force  of  the 
stream  is  to  drive  the  boat  across  the  stream  in  a  cir- 
cular arc,  of  which  the  point  to  which  the  boat  is  fast- 
ened is  the  centre.  By  changing  the  position  of  the 
rope  to  the  opposite  side  of  the  bow,  a  return  to  the 
bank,  whence  the  boat  was  at  first  caused  to  depart, 
is  effected. 

(4.)  Those  in  which  a  continuous  circular  is  con- 
verted into  a  reciprocating  rectilineal  motion  ;  or  a 
reciprocating  rectilineal  motion  into  one  continuous 
and  circular.  Thus,  in  the  apparatus  represented 
by  Fig.  5,  an  eccentric  plate  being  made  to  revolve 
on  a  fixed  axis,  will  cause  a  rod  which  rests  upon  it 
to  rise  and  fall  alternately.  The  plate  may  be  cir- 
cular or  of  any  other  figure  ;  where  it  has  such  a 
shape  as  is  represented  in  the  figure,  it  is  called  a 
heart-wheel. 

In  the  piston  rod  and  crank  of  a  horizontal  steam- 
engine,  the  rectilineal  reciprocating  motion  of  the  for- 
mer gives  a  continuous  circular  motion  to  the  latter. 


20 


PRACTICAL   MECHANICS. 
Fig.  5. 


(5.)  Those  in  which  a  continuous  circular  motion 
generates  another  motion  of  the  same  description. 
For  instance,  a  tight  band  may  be  passed  over  two 
wheels  at  some  distance  from  each  other,  and  if  one 
of  them  be  set  in  motion,  the  friction  between  it  and 
the  band  will  set  the  band  in  motion,  and  its  friction 
on  the  other  wheel  will  move  that  wheel  also.  So 
Fig.  6. 


also  if  teeth  be  cut  on  the  circumference  of  a  wheel, 
and  these  catch  into  the  spaces  between  teeth  cut 


PRACTICAL   MECHANICS. 


21 


upon  another  wheel,  as  in  Fig.  6,  the  motion  of  either 
of  them  around  its  centre  will  give  motion  to  the 
other,  but  in  an  opposite  direction. 

(6.)  Those  in  which  a  continuous  circular  is  con- 
verted into  a  reciprocating  circular  motion,  and  vice 
versa.  Thus,  in  the  scapement  of  the  common 
watch,  a  wheel,  called  a  crown  wheel,  which  is 
caused  to  revolve  continually  by  the  action  of  the 
mainspring,  gives  a  reciprocating  circular  motion  to 
the  verge,  which  forms  the  axle  of  the  balance ;  in 
the  scapement,  represented  Fig.  7,  the  revolution  of 
the  swing- wheel  of  a  clock  gives  motion  to  the  an- 
Fig.  7. 


chor  pallets,  which  are  attached  to  the  crotch  in 
which  the  pendulum  is  inserted ;  and  thus,  in  the  spin- 
ning-wheel, the  foot  applied  to  a  treadle  gives  a  mo- 
tion to  and  fro  in  a  circular  arc,  and  this  motion,  con- 
3 


22  PRACTICAL    MECHANICS. 

veyed  by  a  rod  to  the  crank,  causes  it  to  revolve  con- 
tinuously in  one  direction. 

(7.)  Those  in  which  a  rectilineal  motion  is  convert- 
ed into  an  alternating  circular  motion,  or  a  recipro- 
cating circular  into  a  reciprocating  rectilineal  mo- 
tion. We  have  an  instance  of  the  first  description 
in  the  piston-rod  and  working-beam  of  the  usual  form 
of  steam-engine ;  and  of  the  second  in  the  handle  or 
brake  of  the  common  pump. 

(8.)  An  alternating  circular  motion  may  be  con- 
verted into  another  of  the  same  description,  but  con- 
trary in  direction.  Thus  a  segment  of  a  circle 
which  has  a  reciprocating  motion  may  be  cut  into 
teeth,  which  catch  into  the  spaces  between  teeth 
formed  in  the  segment  of  another  circle. 

6.  In  every  machine  there  are  three  motions  which 
require  to  be  particularly  considered  : 

(1.)  The  motion  of  the  moving  power  itself,  which 
may  not  be  the  same  with  that  of  the  part  of  the 
machine  on  which  it  acts. 

(2.)  The  motion  of  the  part  of  the  machine  which 
is  immediately  acted  upon  by  the  moving  power,  and 
which  is  called  the  impelled  point  of  the  machine. 

(3.)  The  motion  transmitted  by  the  machine,  par- 
ticularly that  of  the  part  by  which  the  work  is  per- 
formed, which  is  called  the  working  point. 

7.  However  great  the  number  of  machines,  and 
however  various  the  purposes  to  which  they  are  ap- 
plied, the  prime  movers  employed  by  mechanicians 
are  but  few  in  number,  and  are  all  natural  agents. 
The  utmost  which  human  art  can  do  is  to  call  into 
action  forces  which  exist  in  a  latent  state,  and  to  di- 
rect and  control  their  action.  Of  the  natural  agents 
which  are  employed  in  practical  mechanics,  the 
most  important  are : 


PRACTICAL    MECHANICS.  23 

(1.)  The  force  of  gravity,  acting  through  the  in- 
tervention of  some  descending  weight. 

(2.)  Tlie  elasticity  of  springs. 

(3.)  The  strength  of  men  and  animals. 

(4.)  Water. 

(5.)  Wind. 

(6.)  The  force  of  the  elastic  vapour  of  water  or 
steam. 

In  addition,  we  use  in  a  few  instances  the  explo- 
sive energy  of  gunpowder.  The  attractions  of  elec- 
tricity, magnetism,  and  chemical  affinity  are  also 
capable  of  setting  bodies  in  motion,  and  might  there- 
fore be  applied  to  drive  machines.  But  the  sphere 
of  action  of  these  forces  is  so  limited  as  to  render  it 
improbable  that  they  can  ever  be  applied  to  any  use- 
ful purpose,  with  the  exception  of  the  electro-mag- 
netic influence.  Of  this  an  application  has  recently 
been  made  which  may  possibly  be  effectual.  The 
alternate  expansion  and  contraction  of  the  air  by 
heat  has  been  applied  to  work  mere  models,  but 
there  are  important  difficulties  in  the  way  of  its  ap- 
plication on  the  large  scale.  We  shall  have  occa- 
sion, likewise,  to  refer  to  a  machine  in  which  the  ac- 
tion of  heat  upon  air  causes  motion. 

Among  the  prime  movers  which  have  been  pro- 
posed, but  have  not  yet  been  brought  into  use,  is  car- 
bonic acid  condensed  into  the  solid  or  liquid  form. 

There  is  also  in  the  continually  varying  pressure 
of  the  atmosphere  a  source  of  power  which  might 
be  applied  in  some  few  instances,  and  it  has  been 
used  for  winding  up  clocks. 

Before  machines  were  invented,  or  while  only 
those  of  the  simpler  descriptions  were  known,  man 
could  apply  no  other  prime  mover  than  his  own 
strength.  The  introduction  and  improvement  of 
complex  machines  has  enabled  him  to  call  into  his 


24  PRACTICAL    MECHANICS. 

service  the  great  natural  agents,  water,  wind,  and 
steam. 

8.  As  no  motion  can  take  place  without  the  ap- 
pUcation  of  an  adequate  force,  so  no  machine  can 
act  unless  driven  by  some  natural  agent.  Neither 
can  any  machine  long  continue  to  work  after  the 
prime  mover  ceases  to  act.  Hence  machines  which 
shall  keep  up  their  own  action,  and  which  have  been 
sought  under  the  names  of  perpetual  motions,  are 
impossible. 

9.  The  action  of  a  prime  mover  depends  not  only 
on  its  own  energy  or  intensity,  but  on  the  velocity 
with  which  it  tends  to  cause  the  impelled  point  of  a 
machine  to  move.  The  product  of  these  two  quan- 
tities is  called  momentum.  The  work  done  is  also 
to  be  estimated  by  the  quantity  of  resistance  over- 
come in  a  given  time,  or  by  the  momentum  of  the 
resistance. 

Under  the  term  resistance  are  included  not  only 
the  useful  work  performed,  but  also  friction  and  all 
other  retarding  forces,  such  as  the  action  of  gravity, 
the  resistance  of  the  air  or  other  medium  in  which 
the  motion  is  performed. 

10.  When  the  momentum  of  the  prime  mover  ex- 
ceeds that  of  the  resistance,  the  machine  is  set  in 
motion,  and  will  move  from  a  state  of  rest  with  ac- 
celerated velocity.  If  the  prime  mover  be  an  at- 
tractive force,  which  acts  with  equal  intensity  upon 
a  body  whether  it  be  at  rest  or  in  motion,  the  ten- 
dency to  acceleration  will  continue.  But  if,  as  is 
more  usually  the  case,  the  prime  mover  act  more 
forcibly  upon  bodies  at  rest  than  upon  bodies  in  mo- 
tion, the  rate  at  which  the  impelled  point  of  the  ma- 
chine is  accelerated  will  diminish  at  each  increase 
of  its  velocity.     This  diminution  in  the  action  of  the 


PRACTICAL    MECHANICS.  25 

accelerating  force  will  continue  until  the  momentum 
of  the  resistance  becomes  equal  to  that  of  the  prime 
mover.  The  motion  of  the  machine  then  becomes 
uniform,  or  will  vary  only  within  certain  limits.  It 
is  said  to  be  in  a  state  of  permanent  working,  and 
equilibrium  exists  among  the  moving  and  resisting 
forces. 

This  species  of  equilibrium  which  occurs  in  the 
motion  of  a  machine  is  called  dynamical, 

11.  When  the  prime  mover  is  of  such  a  nature  as 
to  act  more  forcibly  upon  a  body  at  rest  than  upon 
a  body  in  motion,  a  machine  impelled  by  it  may 
cease  to  do  work  from  two  causes  :  it  may  be  loaded 
with  such  a  resistance  that  it  can  no  longer  move ; 
or  it  may  move  so  fast  as  to  receive  no  new  impulse 
from  the  prime  mover.  Between  these  two  states 
there  will  be  a  velocity  of  the  impelled  point,  with 
which  the  greatest  possible  quantity  of  work  will  be 
performed.  This  most  advantageous  velocity  of  the 
impelled  point  is,  in  most  cases,  one  third  of  the 
greatest  velocity  of  which  the  prime  mover  is  capa- 
ble ;  and  the  resistance  which  will  be  overcome  at 
this  velocity  is  four  ninths  of  that  which  will  stop  the 
motion  of  the  machine  altogether. 

12.  It  is,  in  most  cases,  important  that  the  work  of 
a  machine  shall  be  done  with  a  motion  of  the  utmost 
regularity.  A  tendency  to  irregularity  may  arise 
from  two  causes : 

(1.)  The  prime  mover  may  act  unequally  upon 
the  impelled  point  of  the  machine,  and  yet  vary  with- 
in  certain  definite  limits. 

(2.)  The  prime  mover  may  have  a  tendency  to  in- 
crease or  diminish  in  its  mean  intensity  and  velocity, 
or  the  resistance  may  be  subject  to  variation. 

Each  of  these  cases  has  its  appropriate  remedy. 


26  PRACTICAL    MECHANICS. 

The  first  cause  of  irregularity  may  be  counteracted 
by  a  fly-wheel,  the  second  by  a  governor. 

13.  A  fly-wheel  is  a  heavy  circular  disk,  usually 
of  metal,  to  which  a  great  velocity  is  given  by  the 
action  of  the  prime  mover  transmitted  through  the 
machine.  This  wheel,  like  all  other  bodies,  is  pos- 
sessed of  inertia,  by  which  it  resists  the  action  of 
forces  tending  to  accelerate  it,  and  tends  to  continue 
in  motion  when  the  action  of  the  accelerating  force 
ceases  to  act.  When,  therefore,  the  action  of  the 
prime  mover  is  more  than  equal  to  .the  resistance, 
the  fly-wheel  opposes  its  inertia,  but  still  gradually 
acquires  an  increased  velocity  and  corresponding 
momentum.  When  the  action  of  the  accelera^ng 
force  diminishes,  or  even  ceases  altogether,  the  fly- 
wheel does  not  at  once  lose  its  velocity,  but  parts 
with  it  gradually,  distributing  through  the  other  parts 
of  the  machine  the  excess  of  momentum  it  had  pre- 
viously acquired. 

Although  a  fly  requires  a  part  of  the  moving  force 
to  set  it  in  motion,  and  thus,  in  fact,  adds  to  the  re- 
sistance, it  notwithstanding  frequently  enables  an  ir- 
regular force  to  do  work  that  it  would  otherwise  be 
incapable  of  performing.  Thus,  although  a  man  is 
capable  of  exerting  a  force  equivalent  to  raising 
seventy  pounds,  yet,  when  he  turns  a  winch  or 
crank,  there  is  a  part  of  the  revolution  when  his  ut- 
most force  will  balance  no  more  than  twenty-five 
pounds.  If,  then,  the  resistance  exceed  the  latter 
quantity,  he  will  not  be  able  to  make  the  crank  per- 
form an  entire  revolution,  and,  consequently,  can  do 
no  work  at  all.  If,  however,  a  fly  be  applied  to  the 
crank,  he  will  be  capable  of  working  throughout  its- 
whole  revolution  with  a  force  equivalent  to  the  rais- 
ing of  a  weight  of  thirty  pounds. 

The  effect  of  a  fly-wheel  is  proportioned  to  its 
weight,  its  diameter,  and  its  velocity. 


PRACTICAL    MECHANICS.  27 

14.  Some  engines  require  no  separate  fly-wheel, 
as  they  themselves,  or  some  of  their  working  parts, 
may  act  in  the  manner  of  a  fly.  This  is  the  case  in 
the  water-wheel,  which  will  regulate  its  own  motion 
and  that  of  the  machinery  it  drives.  The  principle 
which  is  employed  in  the  fly-wheel  is  also  used  for 
the  purpose  of  accumulating  the  force  derived  from 
a  long  succession  of  impulses,  and  discharging  it  at 
once  upon  a  given  object. 

The  most  familiar  instance  of  this  application  of 
the  principle  is  to  be  found  in  the  coining  engine. 
This  is  a  screw-press,  worked  by  a  long  lever,  the 
two  extremities  of  which  are  loaded  with  heavy 
weights.  A  rapid  motion  is  given  to  this  lever  by 
the  power  of  men,  who  abandon  it  a  short  time  be- 
fore the  die  is  carried  down  to  the  coin.  At  the  in- 
stant the  die  strikes  the  coin,  the  whole  of  the  force 
which  has  been  communicated  to  the  weight  is  dis- 
charged, and  thus  a  deep  impression  is  produced. 

15.  When  the  intensity  of  the  prime  mover  is  sub- 
ject to  variations  which  are  not  confined  within  fixed 
limits,  or  when  the  machine  may  be  required  to  per- 
form very  different  quantities  of  work,  the  action  of 
the  prime  mover  itself  is  regulated  by  an  apparatus 
called  a  governor. 

A  governor  consists  of  two  heavy  balls,  suspended 
by  means  of  bars  from  a  vertical  axis.  Each  of 
these  bars  is  connected  with  the  axis  by  a  hinge. 
These  bars  form  a  part  of  a  system  of  levers,  by 
which  a  collar  may  be  made  to  move  upon  the  ver. 
tical  axis.  This  axis  derives  motion  from  the  ma. 
chine,  by  which  a  centrifugal  force  is  communicated 
to  the  balls.  This  centrifugal  force  may  acquire 
such  intensity  as  to  overcome  the  gravity  of  the  balls. 
They  will,  in  consequence,  move  outward,  and  thus 
communicate  motion,  through  the  system  of  levers, 


28 


PRACTICAL   MECHANICS. 


to  the  collar  upon  the  axis.  When  the  velocity  di. 
minishes,  the  balls  fall  inward,  and  thus  move  the  col- 
lar in  an  opposite  direction.  The  collar  acts  upon 
an  apparatus  by  which  the  intensity  of  the  prime 
mover  may  be  varied.  Thus,  in  water-wheels,  it 
opens  or  closes  the  shuttle  by  which  water  is  admit- 
to  the  wheel ;  in  steam-engines,  it  works  a  valve  by 
which  the  area  of  the  steam-pipe  is  increased  or  di- 
minished. 

One  of  the  forms  which  the  governor  frequently 
assumes  is  represented  beneath. 

Fig.  8. 


A  natural  agent  which  has  a  tendency  to  acceler- 
ite  the  machine  on  which  it  acts,  may  notwithstand- 
ing be  made  to  give  a  regular  motion,  after  the  ac- 
celeration has  gone  to  a  certain  extent.  Tliis  is  done 
by  calling  into  action  a  resistance  which  increases  in 
intensity  in  a  higher  ratio  than  the  velocity  of  the 


PRACTICAL   MECHANICS,  29 

part  of  the  machine  on  which  it  acts.  Such  a  resist, 
ance,  it  is  demonstrated  in  the  theory  of  mechanics, 
will  finally  render  any  motion  under  the  action  of  a 
constant  accelerating  force  constant. 


II. 

OF   PRIME    MOVERS. 

1.  Of  Weights. 

16.  A  weight  may  be  made  to  give  motion  to  a 
machine,  by  attaching  it  to  a  cord,  which  cord  may 
pass  over  a  wheel  or  be  coiled  upon  a  barrel.  As 
the  descent  of  a  weight  thus  employed  has  a  contin- 
ual tendency  to  acceleration,  it  is  necessary  that  it 
should  be  regulated.  A  regulator  adapted  to  this 
purpose  may  be  formed  by  placing  leaves  or  plates 
of  metal  in  the  direction  of  radii  upon  a  horizontal 
fly-wheel.  As  the  resistance  of  the  air  in  which  the 
fly-wheel  moves  increases  nearly  in  the  ratio  of  the 
square  of  the  velocity,  the  resistance  to  the  motion 
of  the  leaves  finally  becomes  so  great  as  to  counter- 
act any  farther  tendency  to  acceleration. 

This  apparatus  does  not  furnish  a  perfect  regula- 
tor, inasmuch  as  the  density  of  the  air  is  continually 
varying. 

A  better  mode  of  regulating  the  motion  of  a  de- 
scending weight  is  to  be  found  in  the  pendulum. 

A  machine  impelled  by  a  weight  and  regulated  by 
a  pendulum  is  called  a  clock.  Its  structure  will  be 
explained  in  the  proper  place. 

Wherever  absolute  accuracy  in  the  rate  of  the 
motion  is  not  required,  the  fly-wheel  with  leaves  will 


30  PRACTICAL    MECHANICS. 

act  as  a  sufficient  regulator  to  the  force  of  a  descend- 
ing weight.  Its  most  familiar  application  is  in  the 
common  kitchen-jack,  which  is  an  exact  model  of  the 
form  in  which  clocks  were  originally  constructed. 
An  application  of  the  same  principle  on  a  large 
scale  has  been  made  to  counteract  the  tendency  to 
acceleration  of  cars  upon  the  inclined  plane  of  a  rail- 
way. There  is  an  instance  of  this  sort  on  the  rail- 
road of  the  Delaware  and  Hudson  Canal  Co. 

2.  Of  Springs. 

17.  A  spring  is  a  flat  plate  of  steel,  which,  if  bent 
from  the  position  which  is  determined  by  its  origi- 
nal structure,  tends  to  return  to  its  primitive  form. 
The  form  in  which  springs  are  usually  fashioned  is 
that  of  a  spiral  coil,  and  such  springs  are  usually  en- 
closed in  a  cylinder  or  barrel.  This  barrel  is  ad- 
justed around  a  fixed  pin,  to  which  the  inner  end  of 
the  spiral  is  attached ;  the  opposite  end  is  fastened 
to  the  barrel.  The  spring  may  be  wound  up,  or 
caused  to  form  an  increased  number  of  revolutions 
around  the  central  pin,  by  turning  the  barrel.  As 
soon  as  the  force  by  which  the  spring  is  wound  up 
is  withdrawn,  the  spring  tends  to  uncoil  itself,  and, 
in  doing  so,  turns  the  barrel  around. 

The  force  with  which  a  spring  tends  to  uncoil  it- 
self  is  not  constant,  but  is  greatest  at  first,  and  grad- 
ually diminishes,  until  the  spring  is  uncoiled.  If  the 
spring  were  of  equal  elasticity  throughout,  its  force 
would  be  always  exactly  proportioned  to  its  distance 
from  a  state  of  rest.  The  most  frequent  application 
of  the  spring  to  drive  machinery  is  in  the  case  of 
the  v/atch  and  chronometer. 

The  arrangement  of  the  spring  and  barrel  will  be 
understood  from  Fig.  9. 

Springs  are  not  only  employed  for  the  purpose  of 


PRACTICAL    MECHANICS. 
Fig.  9. 


31 


giving  motion  to  machines,  but  also  in  instruments 
for  measuring  the  intensity  of  forces.  From  the 
property  which  has  been  stated,  it  will  be  obvious 
that  an  uncoiled  spring  will  yield  to  the  action  of  a 
force,  until  the  gradually  increasing  tension  is  in  ex- 
act equiUbrium  with  the  intensity  of  that  force.  An 
instrument  intended  for  this  purpose  is  called  a  dyn- 
amometer. The  dynamometer  of  Regnier  is  com- 
posed of  two  springs,  D  E,  Fig.  10,  having  each  the 
shape  of  a  circular  arc,  and  which  are  united  togeth- 
er by  welding  to  them  two  half  rings  of  steel,  Q  Q, 
in  such  manner  that  the  whole  has  the  shape  of  an 
oval.  From  the  middle  of  one  of  the  arcs,  a  gradu- 
ated quadrant,  N  N,  projects,  which  is  fastened  to 
the  spring  at  B  by  a  screw.  The  quadrant  is  divi- 
ded so  that  each  division  shall  represent  a  determi- 
nate weight  acting  upon  a  screw.  The  moveable 
index,  O  F  K,  is  acted  upon  by  a  bent  lever,  E  H  C, 
the  fulcrum  of  which  is  close  to  the  centre  of  the 
quadrant,  and  which  is  attached  to  the  middle  of  the 
arc  D,  opposite  to  that  on  which  the  quadrant  is  fixed. 
The  apparatus  being  so  adjusted  that  the  index  shall 
stand  at  o  when  the  spring  is  not  acted  upon,  weig?- 


32  PRACTICAL    MECHANICS. 

are  suspended  from  it,  and  the  positions  into  which 
the  index  is  brought  by  them  marked  on  the  hmb,  op. 
posite  to  which  the  quantity  of  weight  required  to 
bring  It  into  that  position  is  marked. 
Fig.  10. 


An  eprouvette  for  measuring  the  force  of  gunpow. 
der  by  the  tension  of  a  spring  has  also  been  invent, 
ed.  This  instrument  has  the  form  of  a  graduated 
quadrant.  At  one  end  of  the  quadrant  a  small  cup 
is  fastened,  in  which  the  gunpowder  is  placed  and 
inflamed.  This  cup  is  closed  by  a  flat  plate  of  iron, 
which  IS  pressed  against  its  opening  by  a  spring. 
1  he  plate  yields  to  the  explosive  action  of  the  gun. 
powder,  and  the  distance  to  which  it  recedes  along 
the  arc  is  the  measure  of  the  force. 

Instruments  constructed  on  similar  principles  are 
used  to  measure  the  strength  of  the  fibres  of  flax  and 
hemp,  and  of  the  threads  spun  from  cotton  and  wool. 


PRACTICAL   MECHANICS.  33 

3.  Of  the  Strength  of  Men  and  Animals. 

18.  Animals  may  themselves  be  considered  as 
machines,  planned  by  the  Creator  with  consum- 
mate wisdom,  and  admirably  adapted  to  the  several 
states  and  circumstances  in  which  they  are  destined 

0  exist. 

19.  The  prime  mover  in  animals  is  their  life,  a 
force  whose  origin  and  action  are  to  us  inscrutable. 
This  vital  energy  is  made,  by  the  exercise  of  the  will 
or  volition,  to  act  in  producing  every  variety  of  motion 
of  which  the  animal  is  capable  ;  but  the  manner  in 
which  this  volition  is  transmitted  is  also  beyond  the 
reach  of  our  finite  capacities.  In  obedience  to  the 
will,  the  muscles  contract  or  are  allowed  to  lengthen, 
and  the  contractile  force  is  applied  to  cause  rigid 
parts  of  the  animal  frame  to  turn  upon  the  joints.  In 
vertebrated  animals,  the  muscles  enclose  the  rigid 
parts,  which  are  called  bones.  In  articulated  ani- 
mals, the  muscles  are  enclosed  within  a  jointed  shell, 
to  which  they  give  motion. 

20.  Each  several  motion  of  a  bone  is  produced 
by  the  joint  operation  of  two  muscles,  which  act  in 
opposition  to  each  other,  and  are  hence  called  an- 
tagonists. One  of  these  acts  in  its  contraction  to 
bend  the  joint,  and  is  called  the  flexor  muscle ;  the 
other  tends  to  straighten  the  joint,  and  is  called  the 
extensor. 

By  the  united  action  of  two  or  more  pairs  of  an- 
tagonist muscles,  and  by  the  simultaneous  operation 
of  those  which  act  upon  different  bones,  every  variety 
of  position  and  attitude  of  which  an  animal  is  capa- 
ble is  produced. 

The  muscles  which  give  motion  to  the  limbs 
are  inserted  in  the  trunk  itself,  or  in  limbs  more 


34  PRACTICAL    MECHANICS. 

near  to  the  trunk  than  the  parts  they  are  intended 
to  move.  These  muscles  are  inserted  into  the  hmbs 
to  which  they  give  motion,  at  no  great  distance  from 
the  joint.  Hence  each  separate  bone,  when  moving 
around  the  joint  as  a  fixed  point,  becomes  a  lever  of 
the  kind  ranked  by  mechanics  as  the  third  class. 
But  when  the  extremity  of  the  limb  is  pressed 
against  an  obstacle,  and  the  muscles  act  to  raise  the 
joint,  the  arrangement  becomes  a  lever  of  the  sec- 
ond class. 

In  levers  of  the  third  class,  velocity  is  gained  at 
the  expense  of  power.  But  this  loss  of  power  is  in 
no  case  attended  with  evil  consequences,  for  the  con- 
tractile power  of  the  muscles  is  in  all  cases  adequate 
to  the  exigences  which  the  habits  of  the  animal  de- 
mand. On  the  other  hand,  great  benefit  is  derived 
from  the  superior  degree  of  agiUty  which  is  thus  con- 
ferred, and  there  are  many  cases  where  the  mechan- 
ical action  or  useful  effect  is  to  be  measured  by  the 
square  of  the  velocity,  instead  of  by  the  velocity  sim- 
ply.  In  all  these  cases  a  lever  of  the  third  class  is 
required  for  the  most  advantageous  exertion  of  the 
strength  of  the  muscles.  The  foot  of  man,  on  the 
other  hand,  is  a  lever  of  the  second  class,  and  is  thus 
calculated  to  raise  a  great  weight  to  a  small  height 
by  a  comparatively  small  force.  The  muscles  which 
perform  this  office  are  much  stronger  in  proportion 
than  in  any  other  animal,  and,  accumulated  in  the 
calf  of  the  leg,  add  not  a  little  to  the  beauty  of  the 
human  figure.  These  muscles  are  wrapped  around 
the  heel,  which  they  act  to  raise  by  causing  the  foot 
to  move  around  its  ball  as  a  fulcrum ;  the  weight  of 
the  body  meanwhile  presses  on  a  point  intermediate 
between  the  insertion  of  the  muscles  and  the  point 
around  which  the  motion  is  performed.  Man  is  thus 
enabled  easily  to  maintain,  and  move  in  that  erect 


PRACTICAL    MECHANICS.  35 

posture  for  which  all  the  rest  of  his  structure  is  fit- 
ted. This  posture  cannot  be  assumed  by  the  ani- 
mals which  in  other  respects  approach  most  nearly 
to  the  human  structure.  In  these,  the  powerful  mus- 
cles which  form  the  calf  of  the  leg  in  man  are  slen- 
der and  comparatively  weak ;  thus,  what  in  man  is 
a  firm  support,  becomes  in  them  a  hand.  These  an- 
imals are  hence  called  quadrumana  or  four-handed. 

21.  The  erect  posture  in  man  is  not  assumed  or 
maintained  without  effort.  The  flexor  muscles  of 
the  limbs  are  shorter  than  the  extensors,  and  thus 
the  position  of  the  joints,  when  volition  ceases,  as  in 
sleep  or  death,  is  slightly  bent.  At  the  instant  of 
dropping  asleep,  the  muscles  before  in  action  relax, 
and  if  a  constrained  posture  have  been  assumed  in 
preparing  for  repose,  a  sensation  is  felt  similar  to 
that  of  a  fall. 

The  exertion  required  to  maintain  the  erect  pos- 
ture is  so  great,  that  the  muscles  which  concur  in 
this  effort  have  frequent  need  of  repose ;  this  is  ob- 
tained by  resting  the  weight  unequally  on  the  two 
feet,  and  shifting  it  alternately  from  one  to  the  other. 

22.  In  most  quadrupeds,  the  relation  between  the 
lengths  of  the  flexor  and  extensor  muscles  is  the 
same  as  in  man ;  and  thus,  when  volition  ceases,  the 
joints  bend,  and  the  position  of  standing  cannot  be 
assumed  and  maintained  without  effort.  The  ele- 
phant is  an  exception  to  this  rule.  His  great  weight 
would  demand  a  vast  exertion  of  strength  to  support 
it,  were  the  usual  relation  of  flexors  and  extensors 
preserved.  But  in  this  large  animal  their  relative 
lengths  are  much  more  near  to  equality,  and  the  leg, 
when  volition  ceases,  takes  the  form  of  a  straight 
column.  Hence  this  animal  can  sleep  without  lying 
down. 


36  PRACTICAL    MECHANICS. 

23.  Birds  have  the  power  of  walking  upon  two  feet, 
of  standing  upon  but  one,  even  when  asleep,  and  of 
clinging  to  a  perch  during  sleep,  or  even  after  death. 
These  powers  are  given  by  an  exactly  opposite  ar- 
rangement to  that  found  in  the  elephant.  The  dif- 
ference in  the  length  of  the  extensor  and  flexor  mus- 
cles of  the  foot  is  much  greater  than  in  any  of  the 
mammalia.  In  consequence  of  this,  the  position  of 
the  talons,  when  the  muscles  are  not  exerted,  is  that 
of  the  greatest  curvature.  In  moving  the  foot,  the 
action  of  the  muscles  spreads  the  toes,  and  they  are 
set  upon  the  ground  in  their  most  extended  position. 
The  subsequent  repose  of  the  muscles  tends  to  draw 
the  claws  together,  but  this  tendency  is  counteracted 
by  the  weight  of  the  bird,  and  the  talons  are  thus 
firmly  fixed  upon  the  ground,  and  their  position  is 
the  more  firm  the  less  the  will  of  the  bird  is  exerted. 
Birds  therefore  may  sleep  resting  on  one  or  both 
feet. 

In  birds  which  perch  when  they  sleep,  the  tendons 
which  bend  the  toes  are  the  prolongations  of  muscles 
near  the  body.  These  tendons  therefore  pass  over 
the  intervening  joint,  so  that  whenever  these  joints 
are  bent,  the  tendons  are  put  to  the  stretch,  and  close 
the  foot  mechanically. 

24.  In  the  progressive  motion  of  animals  over  the 
ground  the  useful  effect  of  the  muscular  force  may 
be  resolved  into  two  parts.  By  the  first  of  these  the 
whole  weight  of  the  animal,  and,  consequently,  its  cen- 
tre of  gravity,  is  raised  a  small  distance  at  each  step. 
By  the  second,  the  centre  of  gravity  is  pressed  for- 
ward until  its  line  of  direction  falls  within  a  new 
base,  provided  by  the  forward  motion  of  the  limbs. 

The  first  of  these  motions  is  performed  in  man 
with  great  ease,  in  consequence  of  the  mechanical 
property  of  the  foot  which  has  been  mentioned,  and 


PRACTICAL   MECHANICS.  37 

the  strength  of  the  muscles  of  the  calf  of  the  leg. 
The  second  of  these  motions  is  performed  with  the 
necessary  rapidity,  because  all  the  other  limbs,  as  we 
have  already  stated,  are  levers  of  the  third  class. 

25.  When  a  man  resting  equally  on  both  feet  wish- 
es to  walk,  the  body  is  swayed  towards  one  side  until 
the  weight  rests  wholly  upon  one  of  the  feet ;  the 
other  foot  is  then  Hfled  from  the  ground,  and  carried 
forward  until  a  step  of  the  usual  length  is  taken,  and 
the  foot  again  reaches  the  ground.  While  this  mo- 
tion is  performing  by  the  foot  and  leg,  the  other  leg 
is  slightly  bent,  and  the  muscles  of  the  calf  are  ap- 
plied to  raise  the  centre  of  gravity  to  a  small  height ; 
at  the  same  time,  these,  with  other  muscles,  are  em- 
ployed to  throw  the  body  diagonally  forward,  until  the 
weight  rests  upon  the  foot  which  has  been  in  motion, 
and  is  just  set  down.  The  foot  which  had  remained 
fast  during  the  first  step  is  now  raised  from  the 
ground,  and  a  similar  operation  repeated,  until  it  is 
planted  and  the  weight  of  the  body  rests  upon  it.  In 
running,  the  foot  whence  the  motion  is  performed  is 
raised  from  the  ground  by  a  powerful  exertion  of  the 
muscles,  before  the  other  foot  is  set  down.  In  walk- 
ing, therefore,  both  feet  are  upon  the  ground  together 
at  the  beginning  and  end  of  each  step,  and  one  of 
them  is  always  resting  upon  it ;  while,  in  running,  the 
feet  strike  the  ground  alternately,  and  the  body  is,  in 
the  interval,  thrown  into  the  air. 

26.  A  horse  or  other  quadruped,  when  about  to 
move,  leans  forward ;  his  feet  are  then  raised  in  suc- 
cession. In  walking,  one  of  the  fore  feet,  say  the 
right,  is  first  lifted  and  thrown  forward,  the  lefl  hind 
leg  is  lifted  immediately  after.  A  short  interval  then 
follows,  after  which  the  left  fore  leg  is  raised,  and  al- 
most immediately  followed  by  the  right  hind  leg.    In 

4 


38  PRACTICAL   MECHANICS. 

trotting,  two  diagonally  opposite  feet  are  mised  at 
the  same  instant  of  time,  and,  after  they  reach  the 
ground  together,  the  remaining  two  feet  are  raised 
at  the  same  moment.  In  racking,  where  the  body 
is  swayed  from  side  to  side  during  the  progressive 
motion,  as  in  the  walk  of  man,  the  two  right  feet  are 
raised  in  quick  succession,  and  are  followed,  after 
they  reach  the  ground,  by  the  two  left  feet. 

In  galloping,  the  feet  are  taken  up  one  by  one,  but 
the  right  fore  leg  follows  the  left  fore  leg  at  a  short 
interval ;  the  right  hind  leg  moves  next,  and  is  im.- 
mediately  followed  by  the  left  hind  leg. 

27.  The  motion  of  birds  through  the  air,  or  flying, 
is  performed  by  the  action  of  the  wings  upon  the  air. 
These  are  kept  in  action  by  means  of  powerful  mus- 
cles situated  upon  the  breast  of  the  bird,  and  which 
are  hence  called  pectoral.  By  the  action  of  these 
powerful  muscles  a  rapid  oscillation  is  given  to  the 
wings.  Although  the  velocity  of  this  motion  is  equal 
in  both  directions,  yet  as  the  wing  is  convex  above 
and  concave  below,  it  is  much  more  resisted  in  the 
downward  than  in  the  upward  stroke  ;  the  result  of 
the  two  motions,  therefore,  is  to  raise  the  bird.  Du- 
ring the  downward  stroke  also,  the  great  feathers 
which  compose  the  wing  strike  the  air  directly,  and 
close  upon  each  other  so  as  to  form  a  continuous  sur- 
face ;  while  during  the  upward  stroke  they  meet  the 
air  obliquely,  or,  rather,  by  an  edge,  and  the  air  has  a 
free  passage  between  them.  The  direction  of  these 
motions  is  inclined,  and  thus  the  downward  stroke  is 
not  only  eflicient  in  supporting  the  bird,  but  in  giving 
it  a  progressive  motion.  The  breathing  apparatus 
of  birds  is  so  constructed  that  the  air  they  respire  is 
passed  through  the  quills  and  other  tubes  of  the 
feathers.     By  this  circulation  of  air  the  density  of 


rRACTlCAL    MECHANICS.  39 

the  bird  is  materially  lessened,  and  may  thus  be  sup- 
ported  by  a  less  exertion  of  force. 

In  the  bat,  whose  skeleton  approaches  closely  in 
structure  to  that  of  man,  the  wings  are  membranes 
spread  upon  the  hind  legs  and  the  fingers  or  toes  of 
the  arms  or  fore  legs.  Motion  is  given  to  the  latter 
of  these  by  strong  pectoral  muscles,  as  in  birds.  In 
comparing  the  structure  of  this  animal  with  that  of 
man,  it  will  be  at  once  seen  that  the  latter  has  not 
the  power  of  flying,  even  with  artificial  wings,  in 
consequence  of  a  want  of  strength  in  the  pectoral 
muscles.  We  may  also  see  how  monstrous  and  un- 
natural are  the  figures  intended  to  represent  angels 
or  genii,  in  which  the  wings  are  set  upon  the  shoul- 
ders.  The  flight  of  birds  is  directed  upward,  down- 
ward, or  horizontally  by  the  feathers  of  the  tail. 

The  obliquity  of  the  stroke  of  the  wing  diflTers  in 
diflferent  birds,  and  is  expressly  adapted  to  their  mode 
of  life.  It  is  greatest  in  birds  of  prey,  which  are 
consequently  better  formed  for  horizontal  progress- 
ive motion,  and  is  least  in  birds  which  rise  to  great 
heights  in  a  direction  nearly  vertical. 

28.  Fish,  which  live  in  a  denser  medium,  have 
bodies  whose  mean  specific  gravity  is  the  same  as 
that  of  the  fluid  in  which  they  swim.  In  order  to 
cause  their  ascent  and  descent,  they  are  furnished 
with  a  bladder  filled  with  air,  and  acted  upon  by 
muscles.  When  the  air-bladder  is  compressed  by 
these  muscles,  the  fish  becomes  denser  than  water, 
and  sinks ;  when  the  action  of  the  muscles  ceases, 
the  bladder  dilates,  the  fish  becomes  less  dense  than 
the  water,  and  rises. 

The  air-bladder  is  situated  in  the  lower  part  of  the 
body  of  the  fish^  thus  raising  the  centre  of  gravity 
above  that  of  magnitude ;  the  body,  therefore,  may 
be  easily  overturned.     This  tendency  is  counteract- 


40  PRACTICAL   MECHANICS. 

ed  by  two  fins  situated  on  the  breast.  These  pec- 
toral fins  are  moved  by  muscles  of  little  strength, 
and  have  little  effect  in  giving  progressive  motion. 
For  the  latter,  the  tail  is  the  important  instrument, 
by  an  action  resembling  that  by  which  a  boat  is 
sculled.  In  this  important  motion  the  greater  part 
of  the  muscular  matter  of  the  fish  concurs,  and  the 
two  muscles  of  each  pair  are  equal  in  length,  so  that, 
under  circumstances  of  rest,  the  body  of  the  fish  re- 
mains straight.  The  tail  itself  is  a  large  fin,  whose 
curvature  is  altered  by  muscles,  so  that  it  may  strike 
the  water  under  the  circumstances  best  adapted  for 
progressive  motion. 

When  the  volition  of  the  fish  ceases,  the  muscles 
which  are  on  the  air-bladder  and  the  pectoral  fins 
no  longer  act ;  the  body  of  the  fish,  in  consequence, 
becomes  lighter  than  water,  and  the  slightest  force 
overturns  it ;  a  dead  fish,  therefore,  rises  and  floats 
at  the  surface  belly  upward. 

The  instances  which  have  been  cited  bear  but  a 
small  proportion  to  the  vast  number  which  might  be 
adduced  to  prove  design  in  the  animal  creation,  and 
the  exertion  of  a  consummate  wisdom  in  the  Creator. 
The  deepest  researches  of  mechanical  science  have, 
at  each  step  of  their  improvement,  manifested  more 
clearly  the  skill  with  which  the  machinery  of  the  ani- 
mal frame  has  been  planned  and  adapted  to  its  objects. 
Still  more  wonderful  is  the  mode  in  which  the  vital 
energy  is  made,  under  the  action  of  the  volition,  to  set 
the  complicated  machinery  in  motion  or  restore  it  to 
rest.  This  volition,  however,  has  no  influence  upon 
those  motions  which  are  necessary  for  the  support 
of  the  life  of  the  animal.  If  we  rank  man  highest  in 
the  scale  of  organization,  it  is  not  because  he  is  any 
way  better  adapted  to  the  cricumstances  in  which  he 
is  intended  to  live  than  those  animate  beings  we  con- 


PRACTICAL    MECHANICS.  41 

sider  as  inferior.  When,  however,  we  pass  from  the 
grand  division  of  the  animal  kingdom  to  which  man 
belongs,  we  find  in  animals  we  are  accustomed  to 
consider  as  inferior,  a  delicacy  and  perfection  of 
structure  of  which  even  the  boasted  frame  of  man 
falls  short. 

29.  The  force  of  men  and  animals  may  be  estima- 
ted in  the  weight  they  are  capable  of  raising  through 
a  given  height  in  a  given  time.  Each  individual  an- 
imal will  have  a  different  degree  of  strength,  but  in 
those  of  the  same  species  the  comparison  may  be  di- 
rect, and  the  average  strength  of  a  number  of  indi- 
viduals may  be  used  to  express  that  of  each.  In  com- 
paring the  strength  of  men  with  that  of  animals,  or 
the  strength  of  different  species  with  each  other,  they 
must  be  considered  as  applied  to  do  the  same  kind 
of  work ;  and  the  work  which  animals  are  most  fre- 
quently caused  to  perform  is  that  of  draught.  In 
estimating  the  force  required  in  this  species  of  work, 
the  animal  is  supposed  to  move  forward  upon  a  level 
surface,  drawing  a  cord  to  which  a  weight  is  attach- 
ed, and  that  the  weight  is  drawn  vertically  upward, 
as  might  happen  in  consequence  of  the  cord  being 
passed  over  a  fixed  pulley.  Man  may  also  be  sup- 
posed to  work  in  the  same  manner,  and  thus  their 
respective  strengths  may  be  compared. 

30.  Animals  and  men  are  capable  of  exerting  a 
great  degree  of  strength  when  impelled  by  a  sudden 
impulse,  and  of  moving  for  a  short  time  with  great 
velocities ;  but  such  sudden  and  violent  exertions 
are  followed  by  fatigue  and  exhaustion.  In  estima- 
ting the  force  of  animals,  it  is  therefore  necessary  to 
take  into  account  the  number  of  hours  per  day  during 
which  an  animal  can  work,  without  losing  the  power 
of  recruiting  his  strength  in  the  intervals  of  labour ; 


42  PRACTICAL    MECHANICS. 

and  the  number  of  days  per  year  for  which  such  work 
can  be  performed.  The  maximum,  or  greatest  speed, 
then,  is  to  be  taken,  not  as  that  which  can  be  reached 
for  a  short  space  of  time,  but  as  that  which  can  be 
kept  up  for  the  number  of  working  hours  in  a  day ; 
and  for  the  maximum  resistance,  we  are,  in  Hke 
manner,  to  take  that  which  can  be  strained  against, 
but  not  lifted,  in  working  the  same  number  of  hours. 

The  greatest  force  of  draught  which  a  man  can 
exert  is  taken  at  70  lbs.,  his  greatest  velocity  in  walk- 
ing at  six  feet  per  second,  or  a  little  more  than  four 
miles  per  hour.  By  the  principle  of  §  11,  a  man 
works  to  the  greatest  advantage  in  draught  to  raise  a 
weight  of  Slid  lbs.  with  a  velocity  of  ten  feet  per  sec- 
ond. This  is  equivalent  to  raising  4120  lbs.  through 
the  space  of  one  foot  in  a  minute. 

The  utmost  strength  of  a  horse  in  draught  has  been 
estimated  at  420  lbs. ;  his  utmost  velocity  in  walking 
at  ten  feet  per  second ;  he  will  therefore  work  to  the 
greatest  advantage  in  draught  in  raising  186|d  lbs., 
with  a  velocity  of  S^d  feet  per  second.  This  is  equiv- 
alent to  raising  37,333  lbs.  one  foot  high  per  minute. 

A  man  may  work  at  his  most  advantageous  speed 
for  ten  hours  per  day,  for  several  successive  days  ; 
a  horse  cannot  work  more  than  eight ;  but,  in  both 
instances,  days  of  rest  must  be  allowed  from  time  to 
time,  in  order  to  prevent  a  prostration  of  strength. 
One  day  of  rest  in  every  seven  is  found  to  be  suffi- 
cient to  restore  the  strength  of  animals  and  men, 
working  against  resistances  having  the  foregoing 
maximum  measure,  while  fewer  will  not  answer  the 
purpose  ;  hence  the  institution  of  the  Sabbath  is  one 
of  absolute  necessity  to  the  well-being  of  mankind 
and  the  animals  it  has  domesticated. 

Taking  into  view  the  difference  of  the  number  of 
hours  each  can  advantageously  work  per  day,  the 


PRACTICAL   MECHANICS.  43 

strength  of  a  horse  applied  to  draught  is  usually  es- 
timated as  equal  to  that  of  seven  men. 

The  strength  of  a  horse  is  often  used  under  the 
name  of  a  horse-power,  as  the  unit  in  which  the  force 
of  other  natural  agents  is  estimated.  This  unit  has 
been  sometimes  taken  as  low  as  28,000  lbs.,  some- 
times as  high  as  44,000  lbs.,  each  supposed  to  be  raised 
one  foot  per  minute.  The  estimate  of  this  unit  which 
we  shall  employ  is  33,000  lbs.  raised  one  foot  per 
minute. 

31.  Draught  is  by  no  means  the  most  advanta- 
geous mode  of  exerting  human  strength ;  in  fact, 
there  is  no  mode  in  which  he  can  be  applied  to  that 
purpose,  whereby  he  can  do  more  than  by  the  mere 
exertion  of  the  muscles  of  his  arms  and  hands.  But 
in  bearing  burdens,  the  relation  between  the  strength 
of  a  man  and  that  of  a  horse  becomes  greater  than 
one  seventh.  The  force  of  the  former  applied  to 
draught  is  limited  to  seventy  pounds,  while  he  can 
move  under  any  weight  less  than  twice  his  own. 
Even  when  loaded  with  a  weight  bearing  to  his  own 
the  relation  3  :  4,  he  can  mount  almost  vertically 
upward,  as  upon  a  ladder,  with  a  speed  of  two  feet 
per  second.  A  horse,  on  the  other  hand,  supports 
less  weight  than  he  is  capable  of  drawing,  and  cannot 
carry  even  his  own  weight  up  a  plane  inclined  more 
than  45°  to  the  horizon. 

Men  may  carry  weights  nearly  equal  to  their  max- 
imum force  of  draught,  and  move  under  them  with 
considerable  speed.  Thus  a  Roman  soldier  bore  in 
his  arms  provisions  and  equipments,  sixty  Roman 
pounds,  and  performed  journeys  at  the  rate  of  five 
miles  per  hour.  A  French  grenadier  is  loaded  with 
fifty  French  pounds,  and  marches  at  the  rate  of  three 
miles  per  hour.  The  weights  which  are  borne  by 
persons  habituated  to  that  species  of  labour  are  very 


44  PRACTICAL  MECHANICS. 

remarkable ;  the  most  signal  instances  of  this  appli- 
cation of  strength  are  to  be  found  in  the  porters  of 
Constantinople  and  Bagdad,  the  Gallegos  of  Lisbon, 
and  the  coal-heavers  of  London. 

The  following  facts  will  illustrate  more  fully  the 
force  exerted  by  men  and  horses,  applied  to  different 
kinds  of  labour. 

A  man  trained  to  running  will  pass  through  twen- 
ty-five  feet  in  a  second,  or  move  at  the  rate  of  about 
nineteen  miles  per  hour.  A  race-horse  can  run  forty 
feet  per  second,  or  at  the  rate  of  twenty-seven  and 
a  quarter  miles  per  hour ;  neither  of  them  can  sus- 
tain such  a  degree  of  speed  for  more  than  seven  or 
eight  minutes. 

A  man  will  walk  without  a  load  for  ten  hours  per 
day,  and  perform  a  distance  of  from  twenty-five  to 
thirty  miles ;  a  horse  walks  faster,  but  cannot  con- 
tinue his  labour  as  long  ;  thus  the  distance  performed 
is  about  equal,  and  in  the  long  run  a  man  will  out- 
travel  a  horse.  We  have  ourselves  witnessed  the 
performance  of  natives  of  Massachusetts  in  search 
of  lands,  who  have  for  a  week  together  walked  forty 
miles  per  day,  carrying  a  weight  of  15  or  20  lbs. 

A  man  will  carry  a  weight  of  140  lbs.  on  his  back 
with  a  velocity  of  one  and  a  half  feet  per  second. 
If  he  throw  it  down  and  return  unloaded  for  a  new 
burden,  he  is  capable  of  working  six  hours  per  day. 
Using  the  French  mode  of  expression,  his  daily  work 
is  represented  by  the  number  702. 

A  traveller  may  carry  88  lbs.  on  his  back,  with  a 
velocity  of  two  and  a  half  feet  per  second,  for  seven 
hours  per  day.  The  expression  for  the  daily  work 
is  756. 

A  horse  will  carry  265  lbs.  at  the  rate  of  three  and 
a  half  feet  per  second.  Working  for  eight  hours  per 
day,  the  work  may  be  represented  by  the  number 


PRACTICAL   MECHANICS.  45 

o^OO,  or  for  10  hours  by  4750.  If  he  trot,  the  load 
must  be  diminished  to  177  lbs.,  but  the  velocity  is 
doubled,  and  the  time  of  profitable  labour  does  not  ex- 
ceed seven  hours.     The  number,  therefore,  is  4435. 

A  man  pushing  a  handcart,  and  returning  unload- 
ed, will  transport  220  lbs.  at  the  rate  of  one  and  a 
half  feet  per  second.  The  number  which  repre. 
sents  his  performance  is  1800.  With  a  wheelbarrow 
the  load  is  only  133  lbs.,  and  the  number  1080. 

A  horse  draws  on  a  cart  2000  lbs.  with  a  velocity 
of  three  and  a  half  feet  per  second  for  eight  hours 
per  day,  being  a"  useful  effect  of  27,000.  In  trotting 
he  will  not  draw  more  than  800  lbs.  with  a  velocity 
of  seven  feet,  and  the  duration  of  his  labour  is  di- 
minished to  four  and  a  half  hours  per  day. 

A  man  who  walks  without  a  load  up  a  gentle  slope 
or  staircase,  will  raise  his  own  weight  vertically  up- 
ward at  the  rate  of  six  inches  per  second.  Taking 
his  weight  at  144  lbs.,  and  the  day's  work  at  eight 
hours,  the  number  which  represents  it  is  281.  In 
lifting  a  weight  by  a  cord  passing  over  a  pulley,  the 
useful  effect  is  no  more  than  77  ;  in  carrying  articles 
by  hand  up  an  inclined  plane,  it  is  73 ;  and  if  upon 
the  back,  no  more  than  56.  It  will  therefore  be 
seen,  that  if  a  man  were  to  raise  no  more  than  his 
own  weight,  and  cause  it  to  act  as  a  counterpoise  to 
the  weight  intended  to  be  raised,  he  might  perform 
almost  twice  as  much  work  as  if  he  used  a  pulley  or 
bore  it  in  his  hands,  and  nearly  three  times  as  much 
as  if  he  carried  it  on  his  back  up  a  ladder. 

A  man  who  walks  upon  steps  cut  on  the  circum- 
ference of  a  v/heel,  acting  by  his  weight  upon  its 
horizontal  diameter,  raises  a  weight  of  133  lbs.  at 
the  rate  of  six  inches  per  second.  The  useful  effect 
is  259.  A  man  who  walks,  pushing  a  resistance 
before  him,  as  when  working  upon  a  capstan,  over- 
5 


46  PRACTICAL    MECHANICS. 

comes  a  resistance  of  twenty-seven  pounds  with  a 
velocity  of  two  feet  per  second,  giving  the  useful  ef- 
fect 207.  In  working  on  a  winch,  the  resistance  is 
17^-  lbs.,  the  velocity  two  feet  and  a  half  per  second, 
the  useful  effect  is  173.  In  rowing,  the  useful  effect 
is  275. 

A  horse  working  on  a  capstan  raises  100  lbs.  at 
the  rate  of  three  feet  per  second,  giving  a  useful  ef- 
fect of  1166. 

It  will  therefore  appear  that  the  most  advanta- 
geous manner  in  which  human  strength  has  been  ap- 
plied, is  in  the  act  of  rowing.  As  this  would  be  ap- 
plicable with  difiiculty  to  the  motion  of  a  machine, 
the  next  best  mode  should  be  employed,  which  is  that 
of  causing  them  to  step  upon  a  wheel  immediately 
above  its  horizontal  diameter. 

The  numbers  w^hich  we  have  used  to  express  the 
relation  of  the  several  useful  effects  are  the  units  em- 
ployed by  the  French  writers  on  these  subjects,  and 
denote  the  number  of  cubic  metres  of  water  raised 
to  the  height  of  one  metre  in  a  day.  This  furnishes 
a  less  complicated  mode  of  comparison  than  had  we 
used  the  English  method,  in  which  the  number  of 
pounds  raised  to  the  height  of  one  foot  is  employed. 

4.  Of  Water. 
32.  Water  may  give  a  circular  motion  to  a  ma- 
chine in  three  ways :  by  its  impulse,  by  its  weight, 
and  by  its  reaction. 

!  The  utmost  effect  which  any  stream  of  water  could 
ipossibly  produce,  would  be  equivalent  to  raising  the 
weight  of  the  water  to  the  height  whence  a  heavy 
body  would  fall  in  acquiring  the  velocity  of  the  cur- 
rent. We  shall  take  this  for  the  measure  of  the  me- 
chanical force  of  the  water,  and  compare  with  it  the 
useful  effects  produced  by  the  three  different  methods. 


PRACTICAL    MECHANICS. 


47 


33.  The  apparatus  on  which  water  acts  by  its 
impulse  to  produce  a  circular  motion  is  called  an 
undershot  wheel. 

An  undershot  wheel  is  suspended  upon  a  horizontal 
axis,  and  in  its  usual  form  has  upon  its  circumfe- 
rence a  number  of  floats  or  paddles,  whose  planes 
pass  through  the  axis,  and  which  dip,  in  the  lower 
part  of  their  revolution,  into  a  current  of  water. 
These  paddles  are  usually  known  by  the  name  of 
buckets. 

This  form  is  represented  beneath. 

Fig.  11. 


34.  An  undershot  wheel  may  be  loaded  with  such 
a  weight  as  will  prevent  it  from  turning,  or,  were 
there  no  resistance,  might  acquire  the  whole  velocity 
*  of  the  stream ;  in  neither  case  could  it  do  any  work. 
Its  greatest  or  maximum  effect  is  produced  where 
its  velocity  is  two  fifths  of  that  of  the  stream.  This 
fact  was  first  discovered  in  the  experiments  of  Smea- 


48  PKACTICAL    MECHANICS. 

ton,  and  has  since  been  shown  to  be  consistent  with 
theory.  It  is  also  inferred  from  theory,  that  at  this 
velocity  of  two  fifths,  the  useful  effect  of  the  wheel 
would  be,  to  raise  one  third  of  the  weight  of  the  wa- 
ter which  forms  the  current  to  the  height  whence  it 
must  have  fallen  to  acquire  its  velocity  ;  or,  in  other 
words,  one  third  of  the  mechanical  measure  of  the 
action  of  the  water.  This  last  inference  is  found  to 
v^ary  from  the  truth  in  different  modes  of  placing  the 
floats  upon  the  wheel. 

35.  When  the  action  of  undershot  wheels  was  first 
considered  scientifically,  it  was  inferred  that,  in  order 
that  the  water  should  act  more  advantageously,  no 
float  should  interfere  with  the  flow  of  the  current 
upon  another.  To  fulfil  this  condition,  when  the 
lower  float  is  vertical,  the  preceding  float  should  be 
just  quitting,  and  the  succeeding  float  just  entering 
the  water.  Constructed  in  conformity  with  this  con- 
dition, the  best  effect  was  found  to  be  little  more  than 
one  fourth  of  the  mechanical  measure  of  the  action 
of  the  water.  Smeaton,  in  his  experiments,  found 
that  the  most  advantageous  position  of  the  floats  was 
such,  that  when  one  bucket  was  vertical,  two  others 
should  be  immersed  in  the  water,  a  fourth  entering, 
and  a  fifth  emerging  from  it.  In  the  former  case  no 
more  than  two  floats  can  be  in  the  water  at  the  same 
time  ;  in  the  last  case  there  may  be  four.  With  the 
latter  construction  the  effect  of  the  wheel  becomes 
three  tenths  of  the  mechanical  measure  of  the  action 
of  the  water. 

36.  A  farther  increase  in  useful  effect  may  be 
gained  by  closing  up  the  face  of  the  wheel,  and  ap- 
plying flaunches  or  edges  to  the  two  vertical  sides  of 
the  float ;  the  useful  effect  then  becomes  yVo^^s  of 
the  mechanical  measure  of  the  action  of  the  water. 


PRACTICAL    MECHANICS.  49 

37.  A  still  better  arrangement  is  that  proposed 
Dy  Poncelet,  and  represented  beneath. 

Fig.  12. 


In  this  wheel  the  floats,  instead  of  being  plane  sur- 
faces, are  curved  into  the  form  of  a  portion  of  a 
cyHnder.  By  this  arrangement  the  force  of  an 
undershot  wheel  has  been  doubled,  or  increased  to 
two  thirds  of  the  mechanical  measure  of  the  action 
of  the  water. 

The  following  are  the  laws  which  govern  the  ac- 
tion of  undershot  wheels. 

(1.)  In  a  given  undershot  wheel,  if  the  velocity  of 
the  stream  be  given,  the  useful  effect  is  as  the  quan- 
tity of  water  expended. 

(2.)  In  a  given  undershot  wheel,  if  the  quantity  of 
water  expended  be  given,  the  useful  effect  is  as  the 
square  of  the  velocity. 


50  PKACTICAL    MECHANICS. 

(3.)  In  a  given  undershot  wheel,  if  the  quantity  of 
water  expended  be  given,  the  effect  is  as  the  head  of 
water. 

(4.)  In  a  given  undershot  wheel,  if  the  aperture 
whence  the  water  flows  be  given,  the  effect  is  as  the 
cube  of  the  velocity. 

(5.)  To  estimate  the  force  of  an  undershot  wheel 
in  horse  powers  : 

Multiply  the  number  of  cubic  feet  of  water  expend- 
ed per  minute  by  62^  (the  number  of  lbs.  in  a  cu- 
bic foot  of  water),  and  the  product  by  the  effective 
head  or  height  whence  a  heavy  body  must  fall  to  ac- 
quire the  velocity  of  the  water.  This  product  must 
be  reduced  to  one  fourth  for  the  first  form  of  the  un- 
dershot wheel ;  to  one  third  for  that  on  Smeaton's 
plan  ;  to  thirty-six  hundredths  for  that  with  flaunch . 
es ;  and  to  two  thirds  for  that  with  curved  floats. 
Divide  the  product  thus  reduced  by  33,000,  the  quo- 
tient is  the  horse  power  of  the  wheel. 

38.  In  an  overshot  wheel  a  number  of  buckets  are 
formed  upon  its  circumference,  in  such  a  manner  as 
to  receive  water  at  the  highest  point  of  their  revolu- 
tion,  and  discharge  it  wholly  at  the  lowest.  One 
half  of  the  buckets  is  therefore  loaded  with  water, 
which,  by  its  weight,  causes  the  wheel  to  revolve. 

This  arrangement  may  be  understood  by  inspec- 
tion of  Fig.  13. 

39.  In  addition  to  the  action  of  the  weight  of  the 
water  with  which  half  the  wlieel  is  loaded,  the  stream 
may  act  by  its  impulse  upon  the  upper  bucket ;  and 
it  has  been  attempted  to  investigate  by  theory  at 
what  relation  of  height  between  the  wheel  and  the 
whole  fall,  these  two  modes  of  action  united  would 
produce  the  greatest  useful  effect.  The  inference 
was,  that  the   height  of  the  wheel  shoulfl  be   two 


PRACTICAL  MECHANICS. 

Fig.  13. 


51 


thirds  of  the  fall,  and  that  the  water  should  strike 
with  the  velocity  acquired  in  falling  through  the  re- 
maining third.  Experience  has,  however,  shown 
that  this  opinion  is  fallacious  ;  and  it  is  indeed  obvi- 
ous that  these  two  actions  cannot  be  made  to  concur 
usefully,  because  the  velocity  which  is  best  suited  to 
the  one  mode  differs  from  that  adapted  to  the  most  ad- 
vantageous performance  of  the  other ;  and  because 
the  succession  of  impulses  which  is  given  to  the 
buckets  is  inconsistent  with  the  steady  motion  pro- 
duced by  the  weight. 

The  best  practical  rule  is  that  the  water  shall 
reach  the  wheel  with  a  velocity  little  greater  than 
that  with  which  the  latter  revolves  ;  but  the  former 
velocity  may  be  double  that  of  the  wheel,  without  pro- 
ducing any  important  loss  of  power. 

The  velocity  which  in  practice  is  found  to  be  most 
advantageous  for  the  circumference  of  a  water-wheel, 


52 


PRACTICAL    MECHANICS. 


is  not  less  than  two,  nor  more  than  three  feet  per 
second.  The  water  may  therefore  be  permitted  to 
drop  upon  it  from  the  bottom  of  a  spout  or  flume  of 
not  less  than  four,  nor  more  than  fifteen  inches  in 
depth. 

40.  As  the  water  in  the  upper  bucket  has  no  effect 

in  turning  the  wheel,  it  is  better  to  make  the  wheel 

the  whole  height  of  the  fall,  or  even  somewhat  higher ; 

'the  water  will  be  then  introduced,  as  in  the  figure 

beneath,  into  the  second  or  third  bucket  from  the  top. 

Fig.  14. 


An  additional  advantage  is  gained  by  this  arrange- 
ment, for,  the  channel  or  waste-race,  by  which  the 
water  is  carried  off,  must  generally  be  made  in  a  con- 
tinuation of  the  same  direction  as  that  in  which  the 
water  runs  towards  the  top  of  the  wheel.  When  the 
water  is  admitted  into  the  uppermost  bucket,  the  wa- 
ter  in  the  waste-race  moves  in  a  direction  opposite 


PRACTICAL    MECHANICS.  53 

to  that  in  which  the  lower  part  of  the  wheel  revolves 
and  thus  acts  as  a  resistance.  But  when  the  water 
is  admitted  into  the  second  or  third  bucket  from  the 
top,  the  direction  of  the  current  in  the  waste-race  is 
the  same  as  that  of  the  revolution  of  the  lower  part 
of  the  wheel. 

41.  With  the  most  imperfect  form  of  the  overshot 
wheel,  the  useful  effect  is  never  less  than  two  thirds 
of  the  mechanical  measure  of  the  force  of  the  fall  of 
water.  With  the  most  advantageous  construction,  it 
may  amount  to  eight  tenths  of  that  measure. 

42.  The  ordinary  mode  of  constructing  the  buck- 
ets of  overshot  wheels  is  to  close  the  whole  circum- 
ference of  the  wheel  by  boards  called  the  shrouding. 
To  this  are  applied,  at  equal  distances,  other  boards, 
at  right  angles  to  the  circumference,  which  are  again 
met  by  a  third  set  of  boards,  forming,  with  them,  an 
obtuse  angle.  This  arrangement  is  exhibited  in  the 
figure  on  page  52.  The  buckets  are  sometimes 
made  of  sheet  iron,  in  which  case 
they  are  formed  into  a  regular  curve.  ^^' 

43.  It  is  important  that  the  buck- 
ets should  be  constructed  so  as  to  lose  i 
as  little  water  as  possible ;  hence  a 
section,  such  as  is  here  represented, 
is  better  than  the  first  we  have  de-  | 
scribed.     If,  however,  the  aperture 
of  the  bucket  be  made  too  narrow, 
the  water  will  be  impeded  in  its  en- 
trance by  the  contained  air,  or  may 
be  prevented  from  entering  altogeth- 
er.    This  is  sometimes  obviated  by  I 
means  of  small  tubes,  which  form  a 
communication  between  the  upper 
parts  of  two  adjacent  buckets. 


54 


PRACTICAL    MECHANICS. 


The  power  of  an  overshot  wheel  may  be  calcula- 
ted by  multiplying  together  the  number  of  cubic  feet 
of  water  expended  per  minute,  the  height  of  the  fail 
in  feet,  and  the  constant  number  62 J.  Eight  tenths 
of  this  product  divided  by  33,000  will  give  the  horse 
power  in  the  most  advantageous  case,  and  two  thirds 
of  it  in  the  worst  form. 

44.  When  the  water  is  admitted  into  the  bucket 
which  corresponds  in  position  with  the  horizontal  di- 
ameter of  the  wheel,  or  at  a  point  still  lower,  the  ap- 
paratus is  called  a  breast  wheel.  It  may,  in  the  first 
case,  have  buckets  similar  to  those  of  the  overshot 
wheel.     When  the  water  is  admitted  below  the  hor- 

Fig.  16 


PRACTICAL    MECHANICS. 


55 


izontal  diameter,  the  wheel  is  enclosed  in  a  channel 

which  it  nearly  fills,  and  is  famished  with  floats  as 

represented  in  the  annexed  figure,  instead  of  buckets. 

Fig.  17. 


The  force  of  a  breast  wheel  may  be  calculated  in 
the  same  manner  as  that  of  the  overshot  wheel  of  the 
least  advantageous  structure. 

45.  When  water  is  introduced  by  a  pipe  into  a 
horizontal  box,  and  openings  are  made  on  the  two 
opposite  sides  of  the  box  near  the  ends,  the  pressure 
being  taken  off*at  these  openings,  that  on  an  area  equal 
to  that  of  the  openings,  and  opposite  to  them,  becomes 
sensible ;  and  thus  there  is  a  tendency  to  motion  m  the 
box,  in  a  direction  opposite  to  that  in  which  the  water 
issues  from  the  openings.  If,  then,  the  box  be  support- 
ed in  the  middle  upon  a  pivot,  it  will  be  caused  to  move 
in  a  continuous  circle.     This  motion  is  said  to  be  due 


56  PRACTICAL   MECHANICS. 

to  reaction.  This  arrangement  is  represented  be- 
neath, and  is  known  by  the  name  of  Barker's  Mill, 
from  the  person  who  first  proposed  its  use.     Models 

Fig.  18. 


are  occasionally  constructed  of  the  reacting  wheel,  in 
which  there  are  three  arms  instead  of  two. 

It  was  inferred  from  theory  that  Barker's  Mill  was 
the  most  advantageous  possible  method  of  using  the 
power  of  water,  particularly  when  a  small  stream  is 
to  be  apphed,  which  falls  from  a  great  height.  But 
these  anticipations  have  been  far  from  being  realized 


PRACTICAL    MECHANICS. 


57 


;u  practice.  The  cause  of  this  difference  is  that  the 
pipe,  by  which  the  water  is  admitted,  being  attached 
to  the  revolving  apparatus,  revolves  also,  and  the 
water  within  it  acquires  a  centrifugal  force,  by  which 
its  gravity,  and,  consequently,  its  pressure,  is  dimin- 
i.-^hed. 

46.  In  consequence  of  this  defect.  Barker's  Mill 
has  been  modified  by  admitting  the  water  from  be- 
neath, by  an  aperture  to  which  the  revolving  part 
of  the  apparatus  is  adjusted  by  a  water-tight  joint. 
This  form  of  the  reacting  wheel  is  represented  be- 
neath. 

Fig.  19. 


47.  A  form  of  reacting  water-wheel,  which  works 
beneath  the  surface  of  a  body  of  water,  is  represent- 
ed in  Fig.  20. 

The  water,  introduced  from  beneath  into  the  mid- 
dle of  the  wheel,  acts  upon  floats  inclined  to  its  radii, 
and  emerges  obliquely  into  the  channel  which  sur- 
rounds  the  wheel. 

48.  In  another  form,  the  water  is  conveyed  from 


58 


PRACTICAL    MECHANICS. 

Fig.  20. 


the  centre  of  the  wheel  in  a  number  of  spiral  grooves 
to  the  circumference,  whence  it  issues  in  the  direc- 
tion of  a  tangent.     (See  Fig.  21.) 

Innumerable    other   forms    of  horizontal    water- 


PRACTICAL    MECHANICS.  69 

Fig.  21. 


wheels  have  been  proposed  ;  few  or  none  of  them 
have  advantages  equal  to  those  of  the  overshot  wheel. 

49.  The  use  of  the  overshot  wheel  is  usually  lim- 
ited to  heights  less  than  fifty  feet,  in  consequence  of 
the  expense  of  constructing  wheels  of  great  diameter. 
In  cases  where  the  fall  is  great,  the  best  possible  ar- 
rangement is  that  of  an  endless  chain  revolving  over 
axles,  and  to  which  buckets  like  those  of  an  overshot 
wheel  are  attached. 

The  power  of  this  apparatus  may  be  calculated  as 
in  the  best  form  of  overshot  wheel. 

50.  A  horizontal  motion  is  sometimes  produced 
by  the  direct  impulse  of  water.  The  simplest  wheel 
used  for  this  purpose  is  a  low  cylinder  or  drum,  on 
the  circumference  of  which  a  number  of  plane  leaves 


60 


PRACTICAL    MECHANICS. 


are  adjusted  in  an  inclined  direction.  Such  awheel 
acts  with  even  less  power  than  an  undershot  wheel. 
51.  Abetter  method  consists  in  the  application  of 
curved  spiral  channels  to  the  circumference  of  the 
wheel.  The  power  of  this  form  is  said  nearly  to 
equal  the  best  overshot  wheel. 

Fig  22. 


52.  The  Danaid  is  an  instrument  which  has  re 
cently  been  introduced  in  France,  for  using  the  force 
of  water  conveyed  in  a  pipe.  It  is  formed  of  a  ver- 
tical axle,  to  which  are  adapted  leaves  usually  eight 
in  number.  The  axle,  with  its  leaves,  is  enclosed  in 
a  tub  having  a  circular  hole  in  the  bottom  corre- 
sponding in  dimensions  to  the  axle.  The  pipe  which 
conveys  the  water  is  bent  into  the  tub,  and  closed 
at  the  end.     A  vertical  slit  is  made  on  the  side  of 


PRACTICAL    MECHANICS.  61 

liivi  pipe  whence  the  water  spouts  against  the  leaves, 
and  the  water,  after  having  acted,  is  discharged  by 
the  hole  in  the  bottom  of  the  tub. 

A  section  of  this  instrument  is  given  in  the  an 
iiexed  figure. 

Fig>23. 


5.  Of  the  Wind. 

53.  The  apparatus  by  which  the  wind  is  made  to 
produce  a  rotary  motion  is  called  a  cap  windmill. 

54.  There  are  two  kinds  of  windmill,  the  horizon- 
tal and  the  vertical.  The  parts  of  the  windmill 
which  receive  the  impulse  of  the  wind  are  called 
sails.  In  the  horizontal  windmill,  the  sails  revolve 
in  a  horizontal  direction  around  a  vertical  axis.  In 
the  vertical  windmill,  the  sails  revolve  nearly  in  a 
vertical  plane,  around  an  axis  nearly  horizontal. 

In  the  latter  form,  arms,  usually  four  in  number, 
6 


62  PRACTICAL    MECHANICS. 

are  attached  to  the  axis,  and  at  right  angles  to  it. 
To  these  arms  a  framework  is  attached,  on  which 
the  sails  are  spread.  Were  the  pieces  of  wood 
which  form  this  frame,  placed  in  the  same  vertical 
plane  with  the  four  arms,  the  sails  would  have  no 
other  tendency  than  to  overturn  the  mill.  They  are, 
in  consequence,  inclined  to  this  plane,  and  the  force 
of  the  wind  is  therefore  capable  of  being  resolved 
into  two  parts,  one  of  which  still  tends  to  overturn 
the  mill,  the  other  to  cause  the  arms  to  revolve. 
Experience,  aided  by  theory,  has  shown  that  this  in- 
clination must  neither  be  the  same  with  that  which 
would  be  most  advantageous  if  the  sails  did  not  re- 
volve, nor  constant  at  different  distances  from  the 
axis. 

The  inclination  of  the  sail  to  the  plane  of  the 
arms,  at  the  part  nearest  to  the  axis,  is  made  in  the 
best  mills,  22"^ ;  in  the  middle,  18° ;  at  the  extremities, 
7°.  As  this  inclination  is  the  same,  and  towards  the 
same  direction  of  revolution,  in  all  the  arms  of  the  mill, 
all  the  sails  concur  in  producing  the  rotary  motion. 

55.  The  winds  which  are  applicable  to  the  pur- 
pose of  driving  windmills,  are  such  as  have  velocities 
between  thirty  and  twelve  feet  per  second.  When 
the  velocity  falls  short  of  the  latter  hmit,  a  current 
of  wind  is  found  insufficient  to  perform  any  effective 
work.  When  this  velocity  exceeds  twenty-five  feet 
per  second,  it  becomes  necessary  to  lessen  the  sur- 
face of  the  sails,  in  order  to  prevent  the  arms  from 
being  broken  ;  and  when  it  exceeds  thirty  feet,  no  sail 
can  be  safely  spread. 

It  has  frequently  been  attempted  to  construct  hor- 
izontal windmills,  but  they  have  always  disappointed 
the  projectors.  In  Smeaton's  experiments,  the  effect 
of  the  horizontal  windmill  was  not  found  to  be  great- 
er than  one  eighth  of  the  vertical  windmill  exposing 


PRACTICAL    MECHANICS.  63 

the  same  surface  of  sail.  The  reason  of  this  is,  that 
in  the  vertical  windmill  all  the' sails  concur  in  the  ro- 
tary motion,  while  in  the  horizontal  no  more  than 
one  sail  acts  at  a  time  to  the  greatest  advantage,  and 
half  the  sails  are  constantly  moving  in  a  direction 
contrary  to  that  of  the  wind. 

The  most  important  application  of  wind  is  to  pro 
pel  vessels  by  its  action  on  their  sails.     This  will  bo 
treated  of  hereafter. 

56.  The  useful  effect  of  a  vertical  windmill  is 
about  equal  to  a  pressure  of  one  pound  on  each 
square  foot  of  surface.* 

*  The  laws  which  govern  the  action  of  the  windmill  are  as  fol- 
lows : 

The  velocity  of  the  sails  of  a  windmill,  whether  it  be  loaded  so 
as  to  produce  the  maximum  effect,  or  whether  it  have  no  load,  is 
proportioned  to  the  velocity  of  the  wind.  Within  the  limits  al- 
ready stated  for  the  useful  velocity  of  the  wind,  the  number  of 
revolutions  varies  from  six  to  twelve. 

The  maximum  of  effect  is  produced  when  the  velocity  with 
which  the  extremity  of  the  arms  revolves  is  about  the  same  as  that 
of  the  wind.  When  unloaded,  therefore,  the  most  distant  part 
of  the  sails  revolves  more  rapidly  than  the  air  moves.  The  max- 
imum of  effect  is  produced  with  two  thirds  of  the  maximum  ve- 
locity. 

The  maximum  effect  increases  in  a  ratio  somewhat  less  than 
the  square  of  the  velocity  of  the  wind. 

In  a  given  windmill,  the  load  corresponding  to  the  maximum 
effect  is  nearly  as  the  square  of  the  number  of  revolutions  in  a 
given  time. 

When  a  mill  is  so  loaded  as  to  produce  the  maximum  effect 
with  a  given  velocity,  and  the  velocity  of  the  wind  increases  while 
the  load  remains  unchanged,  the  increase  in  the  quantity  of  work 
is,  for  small  changes  of  velocity,  as  the  squares  of  the  velocity ; 
when  the  velocity  of  the  wind  is  doubled,  the  effects  are  increased 
in  the  ratio  10  :  27i  ;  when  the  increase  in  the  velocity  is  more 
than  2:1,  the  effects  are  increased  only  with  the  velocity  of  the 
wind. 

When  the  sails  of  different  windmills  are  similar  in  number  and 
position,  the  number  of  revolutions  in  a  given  time  is  inversely 
proportioned  to  the  length  of  the  arms. 

The  useful  effect  of  similar  windmills  is  proportioned  to  the 
squares  of  the  length  of  their  respective  arras. 


64  PRACTICAL    MECHANICS. 


6.  Steam. 

57.  Water  boilmg  in  an  open  vessel  reaches  a 
temperature  of  212°,  and  forms  steam  of  the  same 
temperature  with  itself.  The  steam  thus  formed  has 
a  bulk  1694  times  as  great  as  that  of  the  water  which 
is  evaporated  to  form  it,  and  an  elastic  ^orce  or  ten- 
sion equivalent  to  the  pressure  of  the  atmosphere. 
This  pressure  is  estimated  at  fifteen  pounds  on  every 
square  inch  of  surface  exposed  to  it. 

When  water  is  confined  in  a  close  vessel,  it  may 
acquire  a  much  higher  temperature,  and  the  steam 
which  is  formed  has  the  increased  temperature  of  the 
water.  With  the  elevation  of  the  temperature,  the 
tension  of  the  steam  increases,  but  in  a  higher  ratio. 
The  relation  between  the  temperature  and  the  ten- 
sion of  steai^i  is  a  complex  one,  but  it  is  sufficient  for 
our  purpose  to  state  that  the  tension  doubles  very 
nearly  for  every  increase  in  temperature  of  40°. 
Thus  steam  of  the  temperature  of  252°  has  a  ten- 
sion of  about  two  atmospheres  ;  steam  of  292°,  a 
tension  of  about  four  atmospheres. 

58.  When  the  tension  of  steam  exceeds  three  at- 
mospheres, or  v^hen  it  is  capable  of  acting  against 
the  pressure  of  the  atmosphere  with  a  force  of  thirty 
pounds  per  square  inch,  it  is  called  high  steam.  The 
density  of  high  steam  is  greater  than  that  of  low 
steam,  but  does  not  increase  as  rapidly  as  the  tension. 
For  example,  while  the  volume  of  steam  of  one  at- 
mosphere is,  as  we  have  stated,  1694,  that  of  steam 
of  two  atmospheres  is  909,  of  three  atmospheres 
625,  and  of  four  476  ;  instead  of  1694  :  847  :  423^, 
which  these  volumes  would  have  been  had  the  den- 
sity increased  as  rapidly  as  the  tension. 

59.  Steam  is  generated  for  the  purpose  of  being 
applied  as  a  prime  mover,  in   close  vessels  called 


PRACTICAL    MECHANICS.  65 

boilers.  The  only  materials  of  which  boilers  have 
ever  been  made  are  cast  iron,  wraught  iron,  and 
copper.  The  two  latter  are  formed  into  sheets,  and 
boilers  are  constructed  by  riveting  them  together. 
Of  these  materials,  cast  iron  is  the  cheapest,  and 
copper  the  most  costly ;  cast  iron  possesses  the  least, 
and  wrought  iron  the  greatest  strength.  Copper  is 
not  acted  upon  by  the  air,  and  is  not  so  readily  acted 
upon  by  substances  held  in  solution  in  the  water,  as 
iron.  When  the  boiler  is  worn  out,  the  cast  iron  is 
nearly  worthless,  wrought  iron  of  little  value,  but  in 
copper  there  is  no  other  loss  than  the  workmanship 
and  a  small  diminution  of  weight.  The  strength  of 
copper  is  greatest  when  it  is  cold,  and  is  perceptibly 
diminished  even  at  the  temperatures  used  in  genera- 
ting steam.  The  strength  of  wrought  iron  increases 
up  to  temperatures  beyond  those  at  which  steam  is 
usually  employed. 

With  proper  precautions,  wrought  iron  may  be 
made  to  endure  for  a  long  time,  and,  in  consequence 
of  its  comparative  cheapness  and  superior  strength, 
has  almost  wholly  superseded  the  two  other  materials. 

60.  The  principal  cause  of  the  rapid  wear  which 
sometimes  destroys  boilers,  is  to  be  found  in  the 
solid  matter  which  is  conveyed  into  them  along  with 
the  water  with  which  they  are  supplied.  This  solid 
matter  may  be  either  in  the  form  of  mud  and  sand, 
mechanically  mixed  with  the  water,  or  in  solution.  . 
The  water  of  rivers  at  a  distance  from  the  sea  con- 
tains only  the  former  kind  of  impurity ;  and  as  it  is 
continually  injected  with  the  water,  while  none  of  it 
is  carried  off  by  the  steam,  it  will  finally  collect  in 
the  form  of  a  crust  on  the  lower  parts  of  the  inner 
surface  of  the  boiler,  unless  some  means  be  taken  to 
prevent  this  deposite.  Wherever  such  a  crust  is 
formed  upon  a  part  of  the  boiler  exposed  to  heat,  it 


66  PRACTICAL   MECHANICS. 

will  protect  the  metal  of  the  boiler  from  the  cooling 
effect  of  the  water  it  contains ;  the  part  beneath  it, 
if  in  contact  with  air  and  flame,  will  often  burn. 
This  is  more  particularly  the  case  in  a  wrought-irpn 
boiler.  ^^ 

When  a  boiler  is  supplied  with  sea  water,  a  period 
will  arrive  in  its  use,  if  proper  precautions  be  not 
taken,  when  the  double  sulphate  of  lime  and  soda 
will  begin  to  be  deposited.  Common  salt  will  next 
cease  to  be  held  in  solution.  The  crust  thus  formed 
will  nat  differ  in  its  effects  from  the  mechanical  de- 
posite.  Finally,  the  muriates  of  lime  and  magnesia 
will  begin  to  be  deposited.  These  last-named  sub- 
stances are  capable  of  being  decomposed  by  the 
metals,  which  are  corroded  in  the  action,  and  their 
effect  on  copper  is  almost  as  certain  as  upon  iron. 
In  order  to  prevent  danger  from  the  deposite  of  the 
last-mentioned  substances,  it  has  been  usual  to  sus- 
pend the  action  of  a  boiler  fed  with  salt  water  for  a 
day  in  each  week,  in  order  that  it  might  be  cleansed 
and  scraped  out.  It  is  possible,  however,  by  permit- 
ting water  to  flow  from  the  bottom  of  the  boiler  from 
time  to  time,  under  the  pressure  of  the  steam,  to 
prevent  or  remove  these  deposites. 

Even  when  clear  spring  water  is  used,  a  small 
quantity  of  saline  matter,  usually  sulphate  of  lime, 
is  often  present,  and,  although  it  may  be  long  before 
any  deposite  takes  place,  still  it  becomes  necessary  to 
guard  against  any  injury  to  the  boiler  growing  out  of 
this  cause.  For  this  purpose,  it  is  sufficient  to  put 
into  the  boiler  clay,  gum,  or  some  cheap  substance 
which  contains  starch,  and  any  injurious  deposite 
from  ordinary  hard  water  may  be  prevented  for  sev- 
eral months. 

61.  The  strength  of  boilers  to  resist  explosive  ac- 
tion depends  upon  the  tenacity  of  the  material,  the 
thickness  of  the  plates  of  which  they  are  composed, 


PRACTICAL    MECHANICS.  67 

their  figure,  and  the  dimensions  of  their  section. 
Wrought  iron,  as  has  been  seen,  is  the  most  ten- 
acious material,  and  its  tenacity  increases  with  the 
heat. 

In  boilers  of  similar  figures,  the  thickness  of  the 
plates  of  which  they  are  composed  requires  to  be  in- 
creased in  the  ratio  of  the  squares  of  their  lineal  di- 
mensions. Of  all  practicable  figures,  the  cylinder  is 
the  strongest,  and  ought,  if  circumstances  will  permit, 
to  be  employed  wherever  high  steam  is  generated. 

The  diameter  of  a  cylindrical  boiler  ought  not  to 
be  less  than  18  inches,  otherwise  it  becomes  difficult 
to  cleanse ;  and  must  not  be  greater  than  five  feet,  or 
it  would  be  impossible  to  give  it  a  proper  degree  of 
strength  to  resist  an  explosive  action  from  within. 

62.  Upon  the  land,  plain  cylindrical  boilers  set 
in  masonry  are  to  be  preferred.  In  steamboats  and 
for  locomotion,  the  boiler  must  contain  a  chamber 
for  the  fireplace,  and  be  furnished  with  internal  flues, 
for  the  purpose  of  using  all  the  heat  which  can  be 
afforded  by  the  combustion  of  the  fuel.  Of  all 
boilers  yet  brought  into  use  for  these  purposes,  that 
employed  in  many  recent  locomotive  engines  is  to 
be  preferred.  This  form  is  represented  beneath. 
Fig.  24. 


A.  Furnace.     B.  Boiler,    c  c.  Water  level,     dddd.  Tabular 
flues.    E.  Chimney. 


68 


PRACTICAL    MECHANICS. 


The  forms  of  which  boilers  have  been  made  are 
so  various,  that  it  would  occupy  too  much  space  to 
enter  into  the  consideration  of  their  relative  advan- 
tages  and  defects.  Among  those  most  worthy  of 
notice  are  the  original  boiler  of  Watt  and  the  boiler 
for  steamboats.  A  longitudinal  section  of  the  boiler 
of  Watt  is  exhibited  beneath. 
Fig.  25. 


aa  a  a.  Boiler. 

6.  Furnace  and  gratv?  bars. 


c.  Ash-pit. 
d  d.  Flue. 


PRACTICAL    MECHANICS. 


69 


h.  Safety-valve.  I.  Valve  of  feeding  apparatus. 

t.'  Float  of  feeding  apparatus.        m   Cistern  of  do. 

k  k.  Lever  of  do.  r.  Counterpoise  of  damper. 

Fig.  26  is  a  cross  section  of  the  same  boiler,  in 
which  the  same  letters  indicate  the  same  parts  as  \p 
fig.  25.     In  addition, 
/.  Represents  the  steampipe.         e.  Man-hole. 
g.  Steam  gauge.  p.  Self-acting  damper. 

Fig.  26. 


70  PRACTICAL   MECHANICS 

As  a  substitute  for  boilers,  it  has  been  attempted 
to  generate  steam  in  other  ways.  Thus  :  water  has 
been  flashed  into  a  red-hot  vessel,  as  in  the  apparatus 
of  Babcock ;  it  has  been  heated  intensely  in  a  strong 
vessel,  as  in  the  generator  of  Perkins  ;  and  the  flame 
and  heated  air  of  the  furnace  have  been  admitted 
into  the  boiler,  to  aid  the  steam  in  its  action  on  the 
engine,  as  in  the  boiler  of  Bennett.  None  of  these 
methods  have  come  into  general  use. 

63.  The  length  of  the  horizontal  flues  which  sur- 
round or  pass  through  boilers,  ought  to  be  such  that 
the  whole  of  the  flame  may  be  expended  before  the 
gases  proceeding  from  the  furnace  reach  the  verti- 
cal chimney.  If  shorter  than  this,  much  heat  will  be 
wasted.  Nor  should  they  be  much  longer,  other- 
wise the  air  which  enters  the  chimney  will  be  too 
cool  to  maintain  a  sufficient  draught  for  the  combus- 
tion of  the  fuel.  The  length  of  the  flues,  therefore, 
depends  upon  the  description  of  fuel  employed,  being 
greatest  in  such  as  burn  with  much  flame. 

64.  The  quantity  of  steam  generated  depends 
upon  the  area  of  the  surface  of  the  boiler  which  is 
exposed  to  the  fire  and  flame.  Eight  square  feet 
are  sufficient  to  generate  the  steam  required  for  the 
nominal  unit  called  a  horse  power,  when  low  steam 
is  generated,  and  nine  square  feet  must  be  allowed 
in  the  use  of  high  steam. 

65.  The  dimensions  of  the  furnaces  depend  upon 
the  quality  of  the  fuel.  Wood,  being  least  dense, 
requires  that  the  furnaces  should  have  the  greatest 
capacity.  In  respect  to  the  depth  from  the  boiler  to 
the  grate  bars,  this  must  be  such  that  the  fuel  shall 
not  touch  the  metal  of  the  boiler,  and  that  the  flame 
shall  be  developed  before  it  touches  the  surface  of 
the  flue.     All  the  attempts  at  saving  heat  by  keeping 


PRACTICAL    MECHANICS.  71 

a  large  surface  of  metal  in  close  contact  with  the 
fuel  have  failed,  as  has  the  method  of  injecting  water 
into  heated  tubes. 

Boilers  are  liable  to  dangers  of  various  kinds,  and 
for  each  there  ought  to  be  not  only  a  self-acting 
mode  of  relief,  but  also  an  indicator  of  the  approach 
of  the  danger. 

G6.  A  regular  supply  of  water  is  not  only  neces- 
sary to  the  constant  action  of  the  engine,  but  is  also 
of  vast  importance  to  the  safety  of  the  boiler  itself. 

Indeed,  the  most  frequent  cause  to  which  the  ex- 
plosion of  boilers  can  be  traced,  is  a  deficiency  in 
the  water  they  contain.  In  this  case  a  part  of  the 
metallic  surface  which  is  in  contact  with  the  flame 
may  become  dry.  When  this  happens,  the  boiler, 
if  of  iron,  may  become  capable  of  decomposing  water 
and  its  vapour ;  and  to  this  generation  of  gas  fatal 
explosions  have  been  occasionally  attributed.  It 
seems,  however,  doubtful  whether  this  decomposition 
can  take  place  to  such  an  extent  as  to  be  often  pro- 
ductive of  serious  danger.  A  much  more  probable 
explanation  is  to  be  found  in  the  relation  between 
the  density  of  steam  and  its  temperature.  Steam, 
after  it  is  generated,  may  have  its  temperature  raised 
by  the  contact  of  heated  metal.  It  will  now  be  un- 
der circumstances  approaching  to  those  which  occur 
when  it  is  heated  out  of  contact  with  water,  and,  in- 
stead of  becoming  denser  with  the  increased  temper- 
ature, will  become  rarer  and  less  capable  of  working 
the  engine,  unless  that  also  have  a  high  temperature. 
If,  however,  water,  itself  heated,  although  to  a  less 
temperature  than  the  steam,  be  injected  into  the 
space  occupied  by  the  steam,,  the  temperature  of  the 
latter  will  be  but  little  lowered,  and  the  steam  will 
at  once  assume  the  density  and  explosive  energy 
belonging  to  this  new  temperature.     This  formation 


72 


PRACTICAL    MECHANICS. 


of  new  vapour  within  the  original  space  might  be  so 
sudden  that  none  of  the  usual  means  will  suffice  to 
give  vent  to  it,  and  the  boiler  must  give  way. 

Such  injection  of  water  into  a  space  occupied  by 
intensely  heated  steam  may  take  place  on  the  open- 
ing of  the  safety-valve,  or  on  letting  steam  from  the 
boiler  into  the  engine,  in  order  to  set  it  in  motion. 
In  either  case  the  water  will  foam  up,  mixed  witli 
steam,  as  occurs  when  a  culinary  vessel  boils  over. 
The  injection  of  water  from  the  feeding  apparatus 
may  also  produce  a  similar  effect. 

67.  The  height  at  which  water  stands  in  a  boiler 
may  be  indicated  by  gauge-cocks,  or  by  a  water- 
gauge. 

The  gauge-cocks  are  situated  on  one  of  the  ends 
of  the  boiler.  They  are  usually  three  in  number. 
The  uppermost  is  situated  above  the  proper  level  of 
the  water ;  the  second  nearly  at  that  level ;  the  third 


Fig.  27. 


beneath  it.  When  the  first  is 
opened,  if  water  issues  from  it, 
the  boiler  is  too  full ;  when  the 
third  is  opened  and  steam  is- 
sues,  the  water  stands  too  low. 
It  is  necessary,  in  observing 
the  indications  of  these  gauge- 
cocks,  to  allow  time  for  any 
water  which  may  have  lodged 
:  in  the  stopcocks  to  be  blown 
out. 

The  water-gauge,  represent, 
ed  Fig.  27,  is  a  glass  tube,  in> 
sorted  at  its  ends  into  two  tubes, 
proceeding,  one  from  the  top, 
the  other  from  the  bottom  of 
the  boiler.  Water  will,  in  con-' 
sequence,  occupy  this  tube  '  ^ 


PRACTICAL    MECHANICS.  73 

bciaie  height  at  which  it  stands  in  the  boiler  it- 

i>3.  Boilers  may  be  supplied  with  water,  or  fed  by 

i'-acting  apparatus,  with  great  ease  when  they 
generate  low  steam.  In  this  case,  no  more  is  ne- 
cessary than  to  pass  a  tube  through  the  upper  sur- 
face of  the  boiler,  and  permit  it  to  descend  nearly  to 
the  bottom.  A  column  of  water  will  be  supported 
in  this  by' the  excess  of  the  tension  of  the  confined 
steam  over  that  of  the  atmosphere ;  and  as  the  water 
in  the  boiler  is  expended,  some  will  enter  it  from  the 
pipe.  If,  then,  the  pipe  be  kept  full,  the  supply  of 
the  boiler  will  be  exactly  equal  to  the  quantity  evap- 
orated. This  pipe  maybe  kept  full  in  either  of  two 
ways  :  1st,  by  supplying  it  with  water  in  such  quan- 
tities as  to  cause  an  overflow  ;  or,  2d,  by  adapting  it 
to  a  cistern,  with  which  it  communicates  by  a  valve, 
and  causing  the  valve  to  open  whenever  the  level  of 
the  water  in  the  pipe  descends  beyond  the  proper 
limit.  The  latter  object  is  effected  by  a  float  with- 
in the  boiler,  connected  by  a  lever  to  a  valve  in  the 
bottom  of  the  cistern. 

An  apparatus  of  this  description,  as  adapted  to 
Watt's  boiler,  is  shown  in  fig.  25,  on  p.  68. 

In  boilers  which  generate  high  steam,  no  other 
method  of  feeding  has  been  introduced  except  the 
forcing  pump.  As  it  is  impossible  to  make  this  sup- 
ply exactly  the  quantity  of  water  which  is  boiled 
away,  it  is  made  to  furnish  more  than  this  exact 
quantity.  It  can  therefore  be  set  in  action  only  at 
intervals.  Self-acting  apparatus,  to  cause  the  water 
raised  by  this  pump  to  enter  the  boiler  or  run  to 
waste  at  pleasure,  has  been  contrived,  but  has  not 
come  into  extensive  use. 

The  force-pump  is  itself  moved  by  the  engine,  and 
the  feeding  apparatus  is  thus  dependant  on  the  mo- 


74  PRACTICAL    MECHANICS. 

tion  of  that  instrument ;  while,  whether  it  shall  force 
water  into  the  boiler  or  not,  is  wholly  at  the  optior 
of  the  engineer.  A  feeding  apparatus  for  high  stean. 
boilers,  which  shall  be  wholly  independent  of  the  mo- 
tion  of  the  engine  and  of  the  attention  of  the  engi. 
neer,  is  therefore  still  a  desideratum. 

69.  The  danger  which  may  arise  from  the  accu- 
mulation of  steam  under  ordinary  circumstances  is 
pointed  out  by  the  steam-gauge.  This,  in  its  most 
usual  form,  is  a  bent  tube  or  inverted  siphon  of  iron, 
adapted  to  the  pipe  which  conveys  steam  from  the 
boiler  to  the  engine.  The  length  of  each  of  the 
branches  of  this  pipe  must  be  equal  to  the  measure 
in  inches  of  mercury,  of  the  utmost  tension  of  the 
steam  for  which  the  boiler  is  calculated.  The  bend 
of  the  tube  is  occupied  by  mercury,  sufficient  in 
quantity  to  fill  either  branch.  When  the  steam  in 
the  boiler  has  the  tension  of  a  single  atmosphere, 
this  mercury  will  stand  at  the  same  level  in  both 
branches  of  the  tube.  As  the  tension  of  the  steam 
increases,  the  mercury  will  be  forced  up  the  outer 
and  open  branch  of  the  tube,  and  depressed  in  the 
other;  the  difference  in  the  respective  levels  will  be 
the  measure  of  the  tension  of  the  steam.  But  the 
depression  in  one  branch  is  exactly  equal  to  the  ele- 
vation in  the  other.  It  is  therefore  sufficient  to 
measure  the  latter,  which  is  done  by  a  rod  which 
floats  on  the  surface  of  the  mercury.  This  rod  is 
divided  into  inches,  each  of  which  is  equivalent  to 
y'^th  of  an  atmosphere,  or  1  lb. 

There  are  various  other  forms  of  steam-gauge,  all 
of  which  act  on  the  same  principle. 

70.  Any  accumulation  of  steam  beyond  the  pre- 
scribed limit  of  tension,  is  obviated  by  the  safety, 
valve. 


PRACTICAL    MECHANICS.  75 

A  safety-valve  is  a  plate  of  metal  ground  to  the 
shape  of  a  frustum  of  a  cone ;  this  is  fitted  to  a  seat 
of  the  same  form,  adapted  to  the  upper  part  of  the 
boiler.  In  order  to  keep  it  in  its  place,  under  the  ex- 
pansive action  of  the  steam  from  beneath,  it  is  load- 
ed with  a  weight.  This  weight  may  be  either  ap- 
plied directly,  or  through  the  intervention  of  a  lever. 
In  the  latter  case,  the  pressure  on  the  valve  exceeds 
the  weight  as  much  as  one  arm  of  the  lever  is  greater 
than  the  other. 

To  this  pressure  must  be  added  the  weight  of  the 
valve  itself,  and  the  sum,  divided  by  the  number  of 
square  inches  in  the  surface  of  the  valve,  gives  the 
pressure  on  the  valve  per  inch.  Instead  of  a  weight, 
a  spring  steelyard  is  now  frequently  used  to  deter- 
mine the  pressure  on  safety-valves.  This  is  more 
particularly  the  case  in  locomotive  engines. 

71.  The  instrument  best  adapted  to  indicate  when 
the  metal  of  the  boiler  or  the  steam  has,  from  any 
cause,  a  temperature  higher  than  is  consistent  with 
safety,  is  the  common  thermometer. 

72.  Any  risk  of  the  bursting  of  the  boiler  from 
this  cause  may  be  obviated  by  adapting  to  a  seat 
similar  to  that  of  the  safety-valve,  a  plate  of  the  al- 
loy called  fusible  metal.  As  the  same  risk  is  to  be 
apprehended  in  the  flues,  a  part  of  them  may  be 
formed  of  plates  of  lead. 

The  objection  to  these  methods  is,  that,  after  they 
have  acted,  the  boiler  will  be  open,  and  therefore  use- 
less. In  respect  to  the  valves  of  fusible  metal,  this 
objection  is  removed  by  an  invention  of  Professor 
Bache,  of  Philadelphia,  who  encloses  the  valve  of  fu- 
sible metal  in  a  pipe,  which  terminates  in  a  safety- 
valve  of  the  usual  form ;  this  valve  can  be  made  to 
close  the  aperture,  as  soon  as  vent  has  been  given  to 
the  steam,  by  the  melting  of  the  fusible*  metal. 


76  PRACTICAL    MECHANICS. 

73.  Self-acting  dampers  have  been  constructed  to 
regulate  the  combustion  of  the  fuel.  These  are,  as 
yet,  only  applicable  to  boilers  generating  low  steam. 

74.  The  communication  between  the  flues  and  the 
chimney  should  also  be  furnished  with  a  damper 
within  the  control  of  the  engineer  ;  the  ash-pit  of 
the  furnace  ought  to  be  provided  with  doors.  By  the 
use  of  these,  the  heat  may  be  moderated  ;  and  when, 
from  a  deficiency  of  water  in  the  boiler,  the  metal 
has  become  intensely  heated,  the  dampers  and  ash- 
pit doors  should  be  closed,  and  the  temperature  al- 
lowed to  subside  before  any  water  is  injected  by  the 
feeding  apparatus.  However  well  the  safety  appa- 
ratus of  a  boiler  may  have  been  planned  and  con- 
structed, much  still  depends  upon  the  capacity  and 
intelligence  of  the  engineer. 

75.  Boilers,  before  they  are  used,  require  to  be 
proved.  The  first  proof  is  performed  by  forcing  wa- 
ter into  the  boiler  by  an  instrument  similar  to  the 
hydraulic  press.  The  pressure  thus  given  ought  to 
be  four  times  as  great  as  the  elastic  force  of  the 
steam  for  which  the  boiler  is  intended.  The  second 
proof  consists  in  loading  the  safety-valve  with  twice 
the  weight  it  is  intended  to  carry  when  in  use. 
Steam  is  then  generated,  and  heat  continues  to  be 
applied  until  the  safety-valve  opens.  The  latter 
proof  can  hardly  be  considered  sufficient  for  a  low- 
pressure  boiler,  and  would  be  too  severe  for  one  in- 
tended for  very  high  steam.  It  would  probably  be 
better  to  subject  boilers  to  a  proof  by  steam  having 
a  constant  excess  of  three  or  four  atmospheres  be- 
yond that  intended  to  be  employed. 

76.  Steam  is  applied  as  a  prime  mover  in  instru- 
ments which  go  under  the  general  name  of  Steam- 
engines.      The   properties  and   mode  of  action  of 


PRACTICAL    MECHANICS.  77 

these  engines  may  be  best  illustrated  by  considering 
the  steps  which  were  successively  made  in  their  im- 
[jrovement,  until  they  attained  the  state  in  which  we 
now  find  them.  Many  ingenious  persons  were,  at 
d  Terent  times,  engaged  in  investigating  the  mechan- 
iv  il  properties  of  steam,  with  a  view  of  applying  it 
to  useful  purposes  ;  we  shall,  however,  take  notice  of 
iione  but  those  who  planned  engines  which  were  ac- 
Uially  employed  beneficially.* 

77.  The  first  engine  which  was  applied  to  any 
practical  purpose  was  that  of  Savary,  and  it  was 
limited  to  the  single  purpose  of  raising  water.  His 
engine  was  composed  of  an  oval  vessel,  communica- 
ting by  a  tube  with  a  boiler,  in  which  steam  was  gen- 
erated. This  communication  had  the  form  of  a  pipe, 
and  was  furnished  with  a  stopcock,  by  means  of 
which  the  flow  of  steam  from  the  boiler  could  be 
regulated,  or  cut  off  at  pleasure. 

From  the  lower  part  of  the  vessel  a  pipe  proceed- 
ed, which  was  bent  outward  for  a  short  distance,  and 
terminated  in  a  vertical  pipe  descending  to  a  reservoir 
whence  water  was  to  be  raised,  and  rising  to  the 
place  in  which  the  raised  water  was  to  be  collected. 
In  this  pipe  were  situated  two  valves,  both  opening 
upward,  and  placed  on  each  side  of  the  pipe  proceed- 
ing from  the  steam  vessel.  These  valves  divide  the 
vertical  pipe  into  two  parts,  each  of  which  has  its 
separate  and  appropriate  use,  and  we  shall  distin- 
guish these  parts  from  each  other  under  the  names 
of  the  ascending  and  descending  pipes. 

This  arrangement  may  be  understood  by  inspec- 
tion of  Fig.  28. 

*  For  a  more  complete  history,  see  the  author's  "  Treatise  on 
the  Steam-engine." 

7 


PRACTICAL   MECHANICS. 

Fig.  28. 


^:  lir  vX^'-  "  ^,,t-"f  S  pipe,  having  a 

e.    Honzontal  pipe  proceeding  m  n.  Pipe  to  furnish  cold  wa- 

"'am'vlssd™"'  ""'  °'  "■'  '^'  for'^condensing  the^sterm, 

biLdm  vessel.  having  a  stopcock  at  n. 


PRACTICAL    MECHANICS.  79 

The  manner  in  which  this  engine  acts  is  as  fol- 
lows, viz.  : 

The  water  in  the  boiler  having  been  raised  to  a 
temperature  of  212°  or  upward,  the  stopcock  on 
the  steam  pipe  is  opened.  Steam  therefore  flows 
towards  the  steam  vessel,  which  it  finds,  in  the  first 
instance,  full  of  air.  The  steam,  being  lighter  than 
air,  tends  to  occupy  the  upper  part  of  the  steam  ves- 
sel, and  thus  displacing  the  air,  causes  it  to  open  the 
valve  on  the  ascending  pipe,  and  to  flow  out  through 
that  channel. 

The  steam  vessel  having  been  thus  filled  with 
steam,  cold  water  is  permitted  to  flow  over  its  sur- 
face by  opening  the  stopcock  n.  The  valve  on  the 
steam  pipe  is  closed  at  the  same  instant.  The  steam 
being  thus  condensed,  a  vacuum  would  be  formed  in 
the  steam  vessel  were  it  not  that  the  air  in  the  as- 
cending pipe  makes  its  way  through  the  valve  g, 
and  is  followed  by  water,  forced  upward  from  the 
reservoir  below  by  the  pressure  of  the  atmosphere. 

This  first  part  of  the  action  of  Savary's  engine 
resembles  that  of  a  common  pump,  and  is  limited  in 
practice  to  a  height  of  about  twenty-five  feet.  When 
the  whole  of  the  steam  is  condensed,  the  steam  ves- 
sel will  (except  after  the  first  condensation,  when  the 
air  originally  contained  in  the  descending  pipe  will 
be  present)  be  filled  with  water. 

The  flow  of  cold  water  over  the  surface  is  now 
stopped,  the  stopcock  on  the  steam  pipe  opened,  and 
the  steam  again  flows  from  the  boiler.  The  valve 
on  the  descending  pipe  is  at  once  closed,  by  the 
weight  of  the  water  in  the  vessel,  and  the  pressure  of 
the  steam.  If  this  pressure  exceed  that  of  an  atmo- 
sphere, the  valve  on  the  ascending  pipe  is  opened,  and 
the  water  forced  upward  through  that  pipe  to  a 
height  which  will  depend  on  the  tension  of  the  steam 


80  PRACTICAL   MECHANICS. 

generated  in  the  boiler.  If  that  tension  amount  to 
two  atmospheres,  this  height  will  be  thirty-four  feet, 
and  thirty-four  feet  more  for  each  additional  atmo- 
sphere. Savary  did  not  attempt  to  use  steam  of 
greater  tension  than  three  atmospheres,  and  hence 
the  height  to  which  water  was  raised  by  the  two  ac- 
tions of  the  steam  vessel  did  not  exceed  ninety  feet. 
In  order  to  employ  the  steam  which  would  other- 
wise have  been  wasted,  or  would  have  accumulated 
in  the  boiler  to  a  dangerous  degree  of  tension,  a  sec- 
ond steam  vessel  was  employed,  communicating  with 
the  same  boiler  and  the  same  vertical  pipe,  and  the 
two  vessels  were  made  to  act  alternately. 

78.  This  engine  is  liable  to  all  the  objections  which 
will  be  stated  as  applicable  to  that  which  superseded 
it,  as  well  as  to  some  which  are  peculiar  to  itself. 
The  latter  are  :  that  it  is  limited  in  its  action,  rais- 
ing water  no  more  that  ninety  feet ;  and  that,  on  ac- 
count of  its  employing  high  steam,  it  was  very  un- 
safe, in  consequence  of  the  imperfection,  both  of  ma- 
terials and  workmanship,  which  existed  at  the  time 
it  was  in  use. 

79.  These  objections  were  removed  in  the  engine 
of  Newcomen  and  Cawley.  In  this  the  steam  was 
employed  for  no  other  purpose  than  to  form  a  vacu- 
um by  its  condensation,  and  was  required  to  possess 
no  higher  tension  than  a  single  atmosphere.  The  ac- 
tual prime  mover  was  the  pressure  of  the  atmosphere, 
which  was  employed  to  raise  a  weight,  and  this  weight, 
in  its  descent,  worked  a  pump.  The  pump,  being  of 
the  description  called  forcing,  had  no  limit  of  height 
other  than  the  strength  of  the  materials  of  which  it 
was  composed. 

The  form  of  this  engine,  in  its  most  perfect  state, 
is  represented  in  Fig.  29. 


PRACTICAL   MECHANICS. 

Fig.  29. 


81 


A.  Boiler. 

b.  Steam  valve. 

b  c.  Steam  pipe. 

D.  Cylinder  open  at  top. 

e.  Piston  fitted  airtight  to  the 
cylinder. 

ef.  Piston-rod. 

fg.  Chain  by  v/hich  the  piston- 
rod  is  connected  to  the  lever 
beam. 

ghikl.  Lever  or  working  beam, 
having  circular  segments  at 


each  extremity,  in  order  to 
adapt  its  reciprocating  circu- 
lar motion  to  the  rectilineal 
motion  of  the  piston  and  pump 
rods. 

I  m.  Chain  by  which  the  pump- 
rod  is  attached  to  the  lever 
beam. 

m  n.  Pump-rod  loaded  with  a 
weight  at  n. 

opq.  Condensing  pipe. 

p.  Condensing  valve 


82  PRACTICAL    MECHANICS. 

Snifting  valve.  w  x.  Hand   gear.     This    waa 

s  t.  Pipe  and  valve  for  carrying  worked  by  a  frame  or  rack 

off  condensed  steam  and  in-  suspended  from  the  working 

jected  water.  beam. 
V.  Pump. 

In  the  primitve  position  of  the  engine,  the  beam  is 
inclined  under  the  action  of  the  weight  with  which 
the  pump-rod  is  loaded,  and  the  piston  is  in  its  high- 
est position  in  the  cylinder.  The  cylinder  being  full 
of  air,  in  order  to  set  the  engine  in  action  the  steam 
valve  h  is  opened,  and  the  steam,  passing  into  the 
cylinder,  rises  to  the  upper  part,  and  expels  the  air 
through  the  snifting  valve  r.  The  whole  of  the  air 
having  been  expelled,  and  the  cylinder  filled  with 
steam,  the  condensing  valve  j>  is  opened,  and  water 
is  injected.  By  the  stream  of  cold  water  thus  intro- 
duced, the  steam  is  rapidly  condensed,  and  a  vacuum 
formed  beneath  the  piston,  on  which  the  pressure  of 
the  atmosphere  acts  with  sufficient  intensity  to  force 
it  down  to  the  lower  end  of  the  cylinder.  The  con- 
densing valve  is  now  closed,  and  the  steam  valve 
opened.  The  equilibrium  of  pressure  on  the  piston 
being  restored  by  the  admission  of  steam  beneath  it, 
the  weight  on  the  pump-rod  preponderates,  and  raises 
the  piston  to  its  primitive  position.  By  a  second  in- 
jection of  water  and  the  consequent  condensation, 
the  piston  is  caused  to  descend  a  second  time,  and 
again  brought  back  to  its  primitive  position  by  ad- 
mitting steam  beneath  it. 

The  pump  therefore  is  worked  by  a  descending 
weight ;  the  weight  is  raised  by  the  pressure  of  the 
atmosphere ;  the  steam  is  appHed  to  restore  the 
equilibrium  of  pressure  for  the  purpose  of  allowing 
the  weight  to  act,  and  to  form  a  vacuum,  in  order  to 
render  the  pressure  of  the  atmosphere  efficient. 

80.  This  engine  continued  to  be  used  for  many 
years,  during  which  it  was  receiving  continual  im- 


PRACTICAL    MECHANICS.  83 

proveraents,  the  most  important  of  which  was  a 
method  of  opening  and  shutting  the  steam  and  con- 
(Knising  valves  by  the  motion  of  the  engine.  It  as- 
sumed its  most  perfect  form  in  the  hands  of  Smeaton, 
.Vt  length  Watt,  being  employed  to  repair  a  model 
oi'  the  engine  of  Newcomen  and  Cawley,  was  sur- 
prised at  the  great  quantity  of  steam  which  was  used, 
and  the  great  bulk  of  water  required  for  condensation. 
He  was  thus  led  to  make  experiments  upon  the  vol- 
ume of  steam  generated  by  a  given  volume  of  water, 
and  the  volume  of  water  which  the  condensed  steam 
was  capable  of  heating  to  the  boiling  point.  In  the 
course  of  these  experiments  he  discovered  the  latent 
heat  of  steam,  at  about  the  same  time  in  which  the 
general  law  of  latent  heat  was  discovered  by  Black. 

By  this  discovery  the  defects  of  the  engine  of  New- 
comen and  Cawley  became  apparent.  It  was  now 
seen  that  the  interior  of  the  cylinder  and  the  lower 
side  of  the  piston  being  cooled  down  to  a  temperature 
of  about  100°  at  each  condensation,  the  steam  which 
first  issues  from  the  boiler  must  part  with  its  latent 
heat  and  be  converted  into  water,  until  the  whole  is 
heated  up  to  the  temperature  of  212°,  and  thus,  until 
that  temperature  is  reached,  the  equilibrium  on  the 
opposite  sides  of  the  piston  will  not  be  restored.  In 
this  way,  five  times  as  much  steam  is  wasted  as 
would  be  necessary  to  fill  the  cylinder,  and  thus  six 
times  as  much  fuel  is  used  as  would  suffice  to  pro- 
duce the  requisite  effect,  provided  the  expenditure  of 
latent  heat  could  be  prevented. 

81.  In  order  to  save  this  latent  heat,  Watt  pro- 
posed to  use  a  separate  vessel  for  the  condensation 
of  the  steam.  As  this  vessel  would  speedily  fill  up 
with  the  water  injected  for  the  purpose  of  condensa- 
tion, it  became  necessary  to  provide  for  the  removal 
of  that  fluid.     Two  methods  presented  themselves. 


84  PRACTICAL   MECHANICS. 

The  first  consists  in  placing  the  condenser  at  least 
thirty-four  feet  above  a  reservoir  of  water,  with  which 
it  communicates  by  a  pipe.     As  a  column  of  w^ater , 
of  greater  height    than  thirty-four   feet  cannot  be : 
supported  by  the  pressure  of  the  atmosphere,  the  con- 1 
denser  would  thus  be  kept  in  the  state  of  nearly  a ' 
perfect  vacuum.     This  method  is,  however,  limited  | 
in  its  application,  from  the  difficulty  of  finding  situa- 
tions exactly  suited  to  its  use.     It  was  therefore  re- 
jected.     The  other  method  consists  in  adapting  a 
pump  to  the  condenser,  by  which  all  the  matter  that 
vessel  contains  may  be  drawn  out. 

As  a  pump  of  the  usual  construction  is  resisted  in 
its  motion  by  the  pressure  of  the  atmosphere  upon 
the  whole  surface  of  the  piston,  a  head  was  adapted 
to  the  barrel  of  the  pump,  through  which  the  piston- 
rod  was  caused  to  work  in  an  airtight  collar.  A 
spout  was  placed  upon  the  side  of  the  barrel,  and  fur- 
nished with  a  valve,  through  which  the  steam,  air, 
and  heated  water  were  discharged. 

This  pump  goes  by  the  name  of  the  air-pump,  from 
the  resemblance  of  its  structure  to  the  air-pump  of 
Smeaton. 

82.  The  water  discharged  by  this  pump  is  still 
warm ;  it  is  therefore  used  for  feeding  the  boiler. 
For  this  purpose  it  is  received  in  a  small  reservoir, 
whence  it  is  conveyed  by  a  pump  prepared  for  the 
purpose  to  the  feeding  apparatus  of  the  boiler. 

83.  The  water  intended  for  the  condensation  of 
the  steam,  and  injected  into  the  condenser,  is  fur- 
aished  by  a  third  pump.  The  last  two  of  these  are 
known  respectively  by  the  names  of  the  hot  and  cold 
water  pumps. 

84.  Finally,  Watt  proposed  to  use  the  steam  itself 
as  the  prime  mover,  in  the  place  of  the  pressure  of  the 


PRACTICAL   MECHANICS.  85 

atmosphere.  For  this  purpose,  the  piston-rod  was 
made  to  work  through  collars,  in  a  cover  or  head 
adapted  to  the  cylinder.  Steam  was  conveyed  from 
the  boiler  to  the  upper  side  of  the  piston,  where  it 
acted  by  its  pressure,  while  that  beneath  the  piston 
was  passing  into  the  condenser.  The  downward 
stroke  having  been  completed,  the  equilibrium  of 
pressure  upon  the  piston  was  restored  by  means  of  a 
side  pipe  furnished  with  a  valve.  This  pipe  forms, 
when  the  valve  is  opened,  a  communication  between 
the  upper  and  lower  end  of  the  cylinder,  and  the 
weight  adapted  to  the  pump-rod  will  now  preponder- 
ate and  draw  the  piston*  up,  the  steam  passing  freely, 
during  this  upward  motion,  from  the  upper  to  the 
lower  side  of  the  cylinder,  through  the  side  pipe. 

85.  The  valves,  which,  in  the  original  form  of 
Newcomen  and  Cawley's  apparatus,  had  been  worked 
by  hand,  were,  by  a  subsequent  improvement,  worked 
by  a  frame  suspended  from  the  working-beam.  Watt 
retained  this  method  in  his  single-acting  engine,  ma- 
king it  work  his  three  valves,  instead  of  the  two  re- 
quired in  the  atmospheric  engine.  This  method  was 
called  the  plug-frame  and  hand-gear. 

86.  Watt's  single-acting  engine  is  represented  in 
Fig.  30. 

A.  Cylinder.  H.  Airpump. 

b.  Piston.  /.  Hot  water  cistern. 

c.  Steam  valve.  K.  Cold  water  cistern. 

d.  Equilibrium  valve.  L.  Frame  by  which  the  hand 

e.  Eduction  valve.  gear  or  levers  1,  2,  3,  of  the 
F.  Condenser.  valves  c  d  e,  are  worked. 

g.  Jet-pipe  and  injection  valve,  m.  Side  or  equilibrium  pipe. 

N.  Working- beam.  u.  Foot  valve. 

0.  Cold  water  pump.  w.  Delivering  door. 

r.  Steam  pipe. 

87.  The  single-acting  engine  produces  an  alter- 
nating motion,  and  works  only  during  the  descent  of 

8 


86 


PRACTICAL    MECHANICS. 
Fig.  30. 


the  piston.  It  is  therefore  unfit  for  almost  any  pur 
pose  except  pumping.  In  order  to  apply  steam  to 
manufacturing  purposes,  it  was  necessary, 

(1.)  That  the  piston  should  be  forced  upward  as 
well  as  downward. 


PRACTICAL    MECHANICS.  87 

(2.)  That  the  connexion  between  the  piston-rod 
and  the  working-beam  should  be  rendered  rigid, 
and,  at  the  same  time,  allow  the  rectilineal  motion  of 
the  first  to  adapt  itself  to  the  circular  motion  of  the 
other. 

(3.)  That  the  reciprocating  motion  of  the  lever- 
beam  should  be  converted  into  one  continuous  and 
circular. 

The  first  of  these  requisite  changes  was  effected 
by  a  second  side  pipe,  by  the  suppression  of  the  equi- 
librium valve,  and  the  substitution  of  two  new  valves, 
the  one  to  admit  steam  to  the  lower  side  of  the  pis- 
ton, the  other  to  convey  steam  from  its  upper  side  to 
the  condenser. 

The  second  property  was  obtained  by  an  apparatus 
called  the  parallel  motion. 

The  third,  by  substituting  a  rod  moving  on  a  piv- 
ot for  the  pump- rod  and  chain. 

This  rod  had  at  its  opposite  end  a  wheel,  the  teeth 
of  which  caught  in  another  wheel  of  equal  diameter 
and  number  of  teeth,  attached  to  a  fixed  axle.  Upon 
this  axle  was  fastened  a  fly-wheel.  By  this  arrange- 
ment, the  axle  and  fly-wheel  were  caused  to  revolve 
twice  for  each  reciprocating  motion  of  the  piston. 
The  method  had  the  name  of  the  sun  and  planet 
wheel.  In  order  to  control  the  engine  when  the 
I  steam  varied  in  tension,  or  when  the  resistance  was 
subject  to  irregularities,  a  governor  was  added,  driven 
by  a  strap  or  band  placed  over  the  axle  of  the  fly- 
' wheel.  The  engine,  thus  completed  by  Watt,  is  rep- 
resented Fig.  31, 

A.  Cylinder.  g  g.  Condensing  valves. 

h.  Piston.  h  h.  Hand-gear  for  condensing 
ccc.  Parallel  motion.  valves. 

i.  Plug  frame.  i.  Eduction  pipe. 

e  e.  Steam  valves.  K.  Airpump. 
f  f.  Hand-gear  working  steam    I.  Airpump  rod. 

valves.  m.  Condenser. 


88  PRACTICAL    MECHANICS. 

n.  Additional  airpump  (no  long-  s.  Cold  water  pump. 

er  used).  1 1.  Sun  and  planet  wheeL 

o.    Hot  water  cistern.  u.  Connecting  rod. 

p  p.  Levers  moved  by  the  gov-  W  W.  Lever  beam. 

ernor,  to  act  on  throttle  valve,  a?  x.  Fly-wheel. 

q.  Governor.  Z.  Band  which  drives  the  gov. 
r.  Hot  water  pump.  ernor. 

Fig.  31. 


Instead  of  the  sun  and  planet  wheel,  a  crank  is 


PRACTICAL    MECHANICS.  89 

now  universally  used.  The  fly-wheel  not  only  serves 
to  regulate  the  motion,  but  also  to  cause  the  crank  to 
pass  through  both  semicircles.  Were  there  no  fly- 
wlieel,  it  might  merely  oscillate  in  the  same  semi- 
circle. 

In  going  thus  from  one  semicircle  to  another,  the 
crank  is  said  to  pass  the  centre. 

88.  The  importance  of  the  double-acting  engine 
of  Watt  requires  that  we  should  describe  it  more  in 
detail,  and  in  reference  to  the  improved  form  which 
has  since  been  given  it. 

The  steam  is  conveyed  to  the  engine  through  a 
pipe,  in  which  is  situated  a  valve  known  by  the  name 
of  the  throttle  valve.  This  has  the  figure  of  a  cir- 
cular plate,  and  is  suspended  on  an  axle  passing 
through  its  horizontal  diameter.  It  may  therefore 
lie  in  the  direction  of  the  pipe,  when  it  will  oppose 
but  little  resistance  to  the  passage  of  the  steam,  or, 
by  moving  through  a  quadrant,  may  close  the  pipe 
altogether. 

89.  The  steam  passes  through  the  steam  pipe  to 
one  of  the  side  pipes,  and  thence  alternately  to  cham- 
bers situated  at  the  two  ends  of  the  cylinder,  known 
by  the  name  of  steam  chests. 

Each  steam  chest  is  divided  into  three  parts  by 
means  of  two  partitions.  The  middle  part  of  each 
steam  chest  communicates  with  the  cylinder,  the  up- 
per part  with  the  side  pipe  just  spoken  of,  the  lower 
part  with  the  other  side  pipe,  which  is  prolonged  un- 
til it  enters  the  condenser.  The  latter  is  called  the 
eduction  pipe. 

90.  Each  partition  has  a  circular  opening,  ground 
into  a  conical  form,  to  which  a  conical  valve  is  fit- 
ted. When  the  upper  valve  of  either  chest  is  open- 
ed, steam  flows  through  it  into  the  cylinder ;  and 


90  PRACTICAL    MECHANICS. 

when  the  lower  valve  is  opened,  steam  flows  through  [ 
it  from  the  cylinder  into  the  condenser.     The  four 
valves  are  therefore  united  in  pairs,  so  that  a  steam 
and  a  condensing  valve  of  two  separate  steam  chests  ! 
siiall  be  opened  and  shut  together. 

91.  The  valves  were  formerly  moved  by  adapting 
a  rack  to  each  ;  this  rack  was  caught  by  the  teeth 
of  a  segment,  which  was  moved  by  a  lever  passing 
airtight  through  the  sides  of  the  steam  chest.  They 
are  now  usually  moved  by  vertical  rods.  In  the 
top  of  each  steam  chest  a  hole  is  left,  through  which 
the  rod  of  the  upper  valve  passes,  and  the  joint  is  se- 
cured by  a  stuffing-box  or  collar  containing  hemp 
and  tallow.  The  rod  of  the  upper  valve  is  bored  and 
ground,  the  rod  of  the  lower  valve  is  solid,  and  pass- 
es through  the  cavity  bored  in  the  rod  of  the  upper 
valve. 

In  a  more  recent  form,  planned  by  Mr.  Hall,  of 
New- York,  the  valves  do  not  lie  vertically  beneath 
each  other,  and  each  valve  has  a  separate  collar  for 
the  passage  of  its  rod. 

92.  Instead  of  four  conical  valves  placed  in  two 
steam  chests,  and  two  side  pipes,  an  arrangement 
which  goes  by  the  name  of  the  slide  or  D  valve  is 
often  used.  To  construct  this,  a  pipe  having  the 
figure  of  half  a  cylinder  is  adjusted  to  the  side  of  the 
cylinder,  having  its  flat  surface  in  contact  with  the 
latter.  The  steam  passages  enter  into  this  pipe. 
Within  this  another  pipe  is  placed  of  less  length, 
which  closely  fills,  and  is  ground  to  fit  the  outer  pipe 
at  its  two  ends,  and  these  parts  are  at  such  distances 
that  when  the  one  covers  the  steam  passage  nearest 
to  it,  the  other  shall  leave  its  corresponding  steam 
passage  open.  The  intervening  part  of  the  inner 
pipe  does  not  fill  the  outer  pipe.     The  steam  is  ad- 


PRACTICAL    MECHANICS.  91 

mitted  into  the  middle  of  the  outer  pipe,  and  fills  the 
See  between  the  two.  The  inner  pipe  being 
Tveable  by  the  action  of  the  engine,  the  s  earn  wiU^ 
rihis  pipe  ascends  and  descends,  pass  alternately 
^to  the  steam  passages.  Openings  are  cut  in  he 
flat  face  of  the  inner%ipe  of  the  same  size  as  he 
steam  passages,  and  at  such  distances  that,  when  the 
one  corresponds  with  its  adjacent  passage,  the  other 
shall  not.  '^Through  these  openings  the  steam  passes 
alte  natey  to  the  inner  pipe,  and  then  to  the  lower 
part  of  the  outer  pipe  which  is  in  communication 
with  the  condenser. 

93.  The  cylinder  is  usually  made  in  three  pieces: 
namely,  a  hollow  metallic  vessel,  l^^ving  the  figure 
"mporfed  by  its  name,  a  lid  or  cover,  and  a  bottom 
o7  bed.plate.  These  three  parts  are  turned  to  fi 
each  other,  and  firmly  fastened  by  screw-bolts  and 
nuts.  The  joints  are  rendered  tight  by  hemp  coated 
wUh  white  lead  and  oil.  In  the  middle  of  the  cover 
^  an  opening  for  the  passage  of  the  piston  rod,  and 
to  TisTs  adapted  a  stiffing-box  filled  with  hemp  and 

*'' The'  passage  for  <|am  to  and  from  the  upper  side 
of  the  Jiston  is  casTupon  the  ho  low  cylinder ;  the 
lower  passage  is  cast  in  the  bed-plate. 

94  The  piston  is  formed  of  two  pieces,  united  by 
screw-bolts  and  nuts.  Between  these  pieces  is  m- 
terposed  a  packing  of  hemp  coated  with  tallow  As 
tte  packing\vears:  it  may  be  tightened  from  time  to 

''"TheCerS  of  the  piston-rod  is  enlarged  into 
the  figure  of  a  truncated  cone.  This  fills  a  similar 
cavitf  in  the  piston,  and  the  two  are  fastened  by 
drilg  a  key^hrough  an  eye  in  the  piston-rod  im- 
mediatlly  above  the  upper  plate  of  the  piston. 


92  PRACTICAL    MECHANICS. 

The  space  through  which  the  piston  moves,  called 
its  stroke,  is  usually  one  half  more  than  its  own  diam- 
eter. The  cylinder  must  be  as  much  longer  as  will 
leave  room  for  the  piston,  and  prevent  it  from  stri- 
king at  either  end  of  its  stroke. 

This  arrangement  of  steam  pipe,  steam  chests, 
valves,  cylinder,  piston,  and  piston-rod,  will  be  under- 
stood by  inspection  of  Fig.  32. 

Fig.  32. 


A  A.  Cavity  of  cylinder.  g.  Side  pipe;  the  other  is  sup- 

hhb  h.  Piston.  posed  to  be  removed,  in  order 

c  c.  Piston-rod.  to  exhibit  the  steam  chests  and 

d  d.  Steam  passages.  valves. 

e  €.  Steam  valves.  h.  Stuffing-box. 

//.  Condensmg  valves.  i  i  i.  Steam  chests. 

Metallic  packing  is  now  used,  in  preference  to 

hemp  and  tallow,  for  rendering  the  piston  tight.     In 

the  best  form  of  metallic  packing,  the  steam  which 

moves  the  piston  acts  as  a  spring  in  order  to  keep 

the  packing  tight. 


PRACTICAL    MECHANICS.  93 

95.  The  condenser  is  also  a  cylindric  vessel, 
formed  of  three  pieces,  the  lower  of  which  is  pro- 
longed, and  serves  as  the  basis  of  the  airpump.  One 
of  the  side  pipes  terminates  in  the  condenser,  which 
thus  receives  steam  alternately  from  the  opposite 
sides  of  the  piston.  The  condensation  is  performed 
partly  by  keeping  the  surface  of  the  condenser  cool, 
by  immersion  in  a  cistern  of  cold  water,  and  partly 
by  the  injection  of  water.  The  injection  is  effected 
by  a  pipe  passing  from  this  cistern  into  the  conden- 
ser. The  quantity  of  water  injected  is  regulated  by 
a  cock  called  the  injection  valve.  The  pipe  often 
terminates  towards  the  condenser  in  a  nozzle,  pierced 
with  holes  like  the  rose  of  a  watering-pot. 

Another  form  of  condenser  is  now  coming  into 
use,  the  invention  of  Hall.  This  is  composed  of  a 
series  of  pipes,  immersed  in  a  cistern  through  which 
a  current  of  cold  water  continually  flows.  The  wa- 
ter condensed  in  these  pipes  is  pumped  back  into 
the  boiler.  The  waste  caused  by  the  generation  of 
steam  is  therefore  identically  replaced,  and  the  boil- 
er, being  wholly  fed  with  distilled  water,  is  not  liable 
to  the  deposite  of  sediment,  or  of  the  saline  matters 
which  are  finally  crystallized,  whenever  water  con- 
taining them  is  employed. 

96.  The  communication  between  the  condenser 
and  airpump  is  by  a  horizontal  rectangular  passage. 
In  this  is  situated  the  lower  valve  of  the  airpump, 
called  the  foot  valve ;  it  has  the  form  of  a  door  sus- 
pended on  hinges  from  its  upper  side. 

97.  The  airpump  is  closed  at  top,  and  its  pis- 
ton-rod works  through  a  stuffing-box.  Its  piston  is 
called  the  bucket,  and  has  a  valve.  This  is  usually 
of  the  figure  called  a  butterfly  valve,  being  coniposed 
of  two  leaves  attached  by  hinges  to  a  diameter  of 
the  piston. 


94  PRACTICAL    MECHANICS. 

The  water,  air,  &c.,  are  discharged  from  the  air 
pump  by  a  rectangular  spout,  on  which  is  situated  a 
valve  similar  in  form  to  the  foot  valve,  and  which  is 
called  the  delivering  door.  The  water  discharged 
by  this  is  received  in  a  small  cistern  called  the  hot 
water  cistern,  in  order  that  it  may  be  used  while 
warm  to  feed  the  boiler.  Where  Hall's  condenser 
is  used,  the  airpump  forces  the  condensed  water  im- 
mediately into  the  boiler. 

98.  In  the  cold  water  cistern  is  immersed  not 
only  the  condenser,  but  the  airpump  also. 

99.  The  cold  water  cistern  is  supplied  to  overflow- 
ing by  the  constant  action  of  the  cold  water  pump. 

100.  The  hot  water  pump  conveys  the  warm  wa- 
ter to  the  feeding  apparatus  of  the  boiler. 

101.  The  capacity  of  the  condenser  in  engines 
used  for  manufacturing  purposes  has  most  frequently 
been  made  one  eighth  of  that  of  the  cylinder ;  its  usual 
dimensions  being  each  one  half  of  those  of  the  latter. 
In  boat  engines  this  capacity  is  increased  one  half, 
and  the  cold  water  cistern  is  dispensed  with.  The 
airpump  has  the  same  capacity  as  the  condenser. 

The  arrangement  of  condenser,  airpump  and  its 
valves,  cold  and  hot  water  cisterns,  may  be  under- 
stood by  inspection  of  Fig.  33. 

102.  The  state  of  the  vacuum  in  the  condenser  is 
ascertained  by  means  of  the  vacuum  gauge.  This 
is  similar  in  principle  and  construction  to  the  barom- 
eter gauge  of  an  airpump.  An  indicator  has  been 
used  for  the  same  purpose.  This  is  a  piston  sup- 
ported in  a  tube  by  a  spring,  which  is  forced  down 
by  the  pressure  of  the  atmosphere.  The  value  of 
this  spring  is  ascertained  by  experiment. 

103.  In  adapting  the  condensing  engine  to  steam- 


PRACTICAL    MECHANICS. 
Fig.  33. 


«.  Eduction  pipe. 
B.  Condenser. 

c.  Injection  valve. 

d.  Injection  pipe  and  nozzle. 

e.  Foot  valve. 
F.  Airpump. 


g.  Bucket. 

h  h.  Butterfly  valve. 

i  i.  Airpump  rod. 

K.  Delivering  door. 

L.  Hot  water  cistern. 

mmm.  Cold  water  cistern. 


boats,  the  weight  of  the  water  contained  in  the  cold 
water  cistern,  or  the  diminution  of  buoyancy  in  case 
of  its  being  in  free  communication  with  the  water  in 
which  the  vessel  floats,  would  be  a  disadvantage.  In 
order  to  suppress  this  cistern,  without  diminution  of 
the  power  of  condensation,  the  capacity  of  the  con- 
denser is  increased  to  half  that  of  the  cylinder.  The 
hot  water  cistern  also,  instead  of  being  placed  on  one 
side  of  the  airpump,  is  set  upon  it,  and  the  delivering 
door  takes  the  form  of  valves  in  the  lid  of  the  air- 
pump. 

104.  The  working-beam  has  usually  three  times 
'  !  length  of  the  stroke  of  the  piston.     The  piston- 
ioiiied  to  it  by  a  pair  of  thin  bars  called  straps  ; 


96  PRACTICAL    MECHANICS. 

these  terminate  in  collars,  adjusted  to  pivots  or  cen- 
tres on  the  head  of  the  piston  and  the  end  of  the 
working-beam.  From  a  point  halfway  between  that  j 
to  which  these  straps  are  attached  and  the  centre  on  i 
which  the  working-beam  oscillates,  another  pair  of 
straps  is  suspended.  The  two  pairs  of  straps  are 
united  by  a  rod  called  the  parallel  bar,  from  its  hav- 
ing that  position  in  respect  to  the  working-beam. 
Half  the  arm  of  the  working-beam,  the  two  pairs  of 
straps,  and  the  parallel  bar,  have  the  figure  of  a  par- 
allelogram,  whose  sides  are  constant  in  length,  but 
whose  angles  may  change  their  dimension.  In  order 
to  guide  the  motion  of  its  angular  points,  the  angle  of 
the  parallelogram  formed  by  the  parallel  bar  and  the 
second  pair  of  straps  is  connected  by  a  rod  to  a  fix- 
ed point.  This  rod  is  called  the  radius-  bar.  The 
angle  with  which  it  is  connected  is  therefore  caused 
to  describe  an  arc  of  a  circle,  of  the  same  number 
of  degrees  with  that  described  by  the  end  of  the 
working-beam,  but  having  its  convexity  turned  in  an 
opposite  direction.  Three  angles  of  the  parallelo- 
gram, therefore,  describe  circular  arcs,  and  the 
fourth  is  thus  caused  to  describe  a  path  which  does 
not  sensibly  differ  from  a  straight  line.  This  fourth 
point  is  that  where  the  piston-rod  is  attached  to  the 
straps.  The  whole  apparatus  goes  by  the  name  of 
the  parallel  motion.  There  is  another  point  in  this 
system  of  bars  and  straps  which  also  describes  a 
straight  line.  This  point  is  situated  in  the  second 
pair  of  straps,  where  a  line  drawn  from  the  centre  of 
motion  of  the  working-beam  to  the  point  where  the 
first  pair  of  straps  is  attached  to  the  piston  rod,  cuts 
the  second  pair  of  straps.  To  this  point  the  rod  of 
the  airpump  is  attached.  The  rods  of  the  cold  and 
hot  water  pumps  do  not  require  a  parallel  motion ; 
they  are    attached    by  collars   to  gudgeons  in   the 


PRACTICAL    MECHANICS.  97 

working-beam,  at  distances  of  one  third    and  one 
half  of  its  length  from  its  centre. 

The  parallel  motion  is  exhibited  in  Fig.  34. 
Fig.  34. 


a  a  a.  Represents  half  the  work-  i  e.  Parallel  bar. 

ing-beam.  e  f.  Second  pair  of  straps. 

b  i.  Straps  which  connect  the  e  h.  Radius  bar. 

'  working-beam  to  the  piston-  ^.  Centre  to  which  the  air-pump 
.  rod.  rod  is  applied. 

For  the  parallel  motion,  a  cross-head  upon  the  pis- 
ton-rod, working  in  two  vertical  guides,  is  now  often 
substituted.  This  method  is  the  only  one  now  used 
in  this  country,  and  is,  for  many  reasons,  superior  to 
the  parallel  motion. 

105.  The  working-bearn  was  at  first  of  wood,  but 
is  now  usually  made  of  cast  iron.  The  centre  of  its 
own  motion,  with  those  of  the  parallel  motion  and 
pump-rods,  are  solid  cylinders  of  wrought  iron,  or 
steel,  carefully  turned,  and  passed  through  the  work- 
ing-beam. 

106.  In  many  American  engines  the  working- 
beam  is  a  frame  of  cast  iron,  surrounded  by  a  strap 
of  wrought  iron,  of  the  figure  of  a  lozenge.  In  this 
construction,  a  greater  degree  of  strength  is  obtained 


98 


PRACTICAL   MECHANICS. 


with  less  weight  of  material.  The  connecting  rod 
of  English  engines  has  a  length  equal  to  twice,  and 
in  American  engines  three  times  the  stroke  of  the 
piston.  The  effective  length  of  the  crank  is  half  the 
stroke  of  the  piston. 

107.  The  crank  is  compelled  to  describe  a  corn- 


Fig.  35. 


plete  circle  instead  of  oscillating, 
as  it  otherwise  might,  in  the  same 
semicircular  arc,  by  adapting  a 
heavy  fly-wheel  to  the  axle  of  the 
crank. 

108.  The  method  of  working 
valves  by  a  plug-frame  suspended 
from  the  lever  beam,  or  by  motions 
taken  from  it,  has  been  in  a  great 
degree  superseded.  The  plan 
most  frequently  adopted  at  present 
is  composed  of  the  eccentric  and 
the  tumbling  shaft.  The  tum- 
bling shaft  has  the  shape  of  a  crank 
and  its  axle ;  cams  are  formed  up- 
on the  latter,  which,  by  means  of 
rods,  serve  to  open  valves  when  of 
a  conical  figure.  These  valves 
shut  by  their  own  weight  when  the 
action  of  the  cam  ceases. 

The  construction  of  the  eccentric 

A.  Eccentric  opening,  through  which  the 
axle  of  the  fly-wheel  passes. 

B.  Iron  plate  of  a  circular  shape. 

c.  Iron  ring  embracing  the  eccentric  plate. 

d.  Triangular  frame. 

g.  Notch  falling  upon  a  part  of  the  tumbling 

shaft. 
g  h.  Crank  of  the  tumbling  shaft. 
/.  Connecting  rod  by  which  a  slide  valve 

may  be  worked. 


PRACTICAL    MECHANICS.  99 

is  as  follows :  a  circular  plate  is  pierced  by  a  circu- 
lar hole,  whose  centre  is  not  the  same  as  that  of  the 
plate  itself.  This  plate  is  wedged  to  the  axle  of  the 
crank,  and  revolves  with  it.  The  plate  is  surrounded 
by  a  circular  ring,  which  does  not  revolve  with  it.  To 
this  ring  two  bars  are  attached,  forming  two  sides  of 
III  isosceles  triangle,  and  the  frame  thus  formed  is 
strengthened  by  braces.  During  the  revolution  of 
the  plate  with  the  axle  of  the  crank,  the  end  of  this 
iriangular  frame  has  a  reciprocating  motion.  This 
motion  is  communicated  to  the  tumbling  shaft  by 
(brming  a  notch  near  the  end  of  the  eccentric ;  this 
notch  drops  on  the  crank  of  the  tumbHng  shaft. 

An  eccentric  is  figured  on  the  opposite  page. 

109.  Watt  was  in  the  habit  of  using  steam  having  a 
tension  about  one  sixth  greater  than  an  atmosphere, 
say,  capable  of  exerting  a  pressure  of  17^  lbs.  per 
square  inch.  At  this  tension  he  found  that  the  effi- 
cient pressure  on  the  piston  was  no  more  than  10  lbs. 
His  rule  for  estimating  the  power  of  a  double-acting 
condensing  steam-engine  is  therefore  as  follows: 
i  Multiply  together  the  area  of  the  piston,  the  length 
of  stroke,  the  number  of  strokes  per  minute,  and  the 
constant  number  10  ;  divide  the  product  by  33,000. 
The  quotient  is  the  horse-power. 

For  each  horse-power  the  evaporation  of  a  cubic 
foot  of  water  per  hour  will  suffice,  and  the  consump- 
rion  of  bituminous  coal  will  be  10  lbs.  per  hour  to 
each  horse-power  of  the  engine. 

This  rule  was  founded  on  the  belief  that  there  was 
a  loss  of  power  equivalent  to  7  J  lbs.  per  square  inch, 
I  in  consequence  of  the  imperfection  of  the  vacuum  in 
I  the  condenser,  the  friction,  and  the  obliquity  of  action 
I  in  the  crank.  Now,  the  tension  of  vapour  corre- 
sponding to  the  usual  temperature  of  condensation  is 
never  more  than  2  lbs.  per  inch  :  the  crank,  from 


100  PRACTICAL   MECHANICS. 

the  fact  that  its  most  favourable  positions  correspond 
with  the  maximum  action  of  the  steam,  and  its  least 
favourable  positions  with  an  absolute  cessation  in  the 
flow  of  the  vapour,  causes  but  a  small  loss  by  obliquity ; 
and  the  friction  is  far  from  being  sufficient  to  make 
up  the  difference.  The  rule  of  Watt  is,  notwith- 
standing, correct  for  the  cases  to  which  his  observa- 
tion was  limited,  namely,  of  a  piston  moving  with  a 
velocity  of  from  200  to  250  feet  per  minute.  At , 
other  velocities  the  rule  would  be  untrue.  The  ac- 
tual cause  of  the  difference  between  the  efficient 
pressure  on  the  piston,  and  the  tension  of  the  vapour 
on  the  boiler  after  the  friction  has  been  allowed  for, 
is  the  fact  which  has  been  referred  to  in  the  intro- 
duction, that  all  forces,  except  such  as  are  constant, 
exert  less  power  upon  a  body  in  motion  than  they  do 
upon  a  body  at  rest ;  and  in  the  case  under  consider- 
ation, if  we  were  to  suppose  the  piston  to  be  moving 
with  the  same  velocity  as  the  steam  was  capable  of 
following  it,  no  pressure  whatever  would  be  exerted. 
This  simple  and  obvious  fact  has  hitherto  escaped 
the  notice  of  all  writers  on  the  steam-engine.  The 
mathematical  investigation  of  the  pressure  upon  the 
piston  due  to  a  given  tension  of  steam  in  the  boiler 
and  a  given  velocity  of  the  piston,  would  be  attended 
with  considerable  difficulty.  The  Ch.  de  Pambour, 
by  considering  the  question  in  another  point  of  view, 
has  been  led  to  formulae  which  are  capable  of  ex- 
pressing the  relation  between  the  tension  of  steam  in 
the  boiler,  the  state  of  the  vacuum  in  the  condenser, 
the  velocity  of  the  piston,  and  measure  of  the  work 
which  may  be  performed.* 

*  The  foundation  of  Pambour's  new  theory  of  the  steam-en- 
gine rests  upon  the  following  equation :  in  which  <S  is  the  quan- 
tity of  water  evaporated  per  minute  ;  m  the  ratio  of  the  volume  of 
steam  generated  under  the  given  pressure  P  in  the  boiler,  to  that 
of  the  water,    m  »S  is  therefore  the  volume  of  steam  formed  pei 


PRACTICAL    MECHANICS.  101 

The  rule  of  Watt,  however,  has  been  so  long  ii 
ise  that  it  will  probably  be  retained,  not  as  a  mode 
jf  measuring  the  duty  of  a  steam-engine,  but  as  one 
ny  which  an  engine  may  be  described  in  contracts 
between  the  purchaser  and  the  maker.  The  denom- 
ination in  horse-power  has  therefore  not  been  chan- 
ged, although,  by  an  improvement  in  the  mode  of 
using  steam,  the  duty  has  been  increased  in  some 
cases  more  than  fourfold. 

The  duty  of  an  engine  is  estimated  in  the  weight 
which  can  be  lifted  one  foot  by  the  combustion  of  a 
bushel  of  coals.  In  Watt's  first  engines  the  duty 
was  20  millions  of  lbs.  By  the  introduction  of  the 
expansive  action  of  steam,  the  duty  has  been  raised, 
in  some  instances,  to  more  than  90  millions. 

110.  Steam  of  greater  tension  may  be  used  in 
the  same  manner  as  low  steam  in  the  condensing 
engine.  To  do  this  in  a  given  engine  would  require 
an  increase  in  the  fire  surface  of  the  boiler,  and,  as 
the  density  increases  with  the  tension,  though  in  a 
less  ratio,  the  advantage  gained  would  not  be  equiv- 
alent to  the  additional  expenditure  of  fuel.  A  given 
engine  might  thus  be  made  to  do  more  work,  but  the 
extra  work  would  cost  more  than  if  performed  with 
an  engine  of  increased  size.     There  would  also  be  a 

minute  in  the  boiler  ;  a  is  the  area  of  the  piston,  v  its  velocity,  and 
R  the  resistance  of  the  load.    Then, 

m  S       P 

whence  we  obtain  for  the  resistance  which  may  be  overcome, 

a  V 
and  for  the  quantity  of  water  to  be  evaporated, 
^    a  V  R 
m  Jr 
9 


102         PRACTICAL  MECHANICS. 

difficulty  in  keeping  up  a  vacuum,  unless  the  size  ol 
the  condenser  and  the  power  of  the  airpump  were 
increased. 

111.  If,  however,  the  communication  between  the 
boiler  and  engine  be  made  intermitting,  and  the  steam 
flow  only  during  a  part  of  each  stroke,  the  tension  of 
the  steam  generated  in  the  boiler  will  be  necessarily 
increased,  even  in  higher  proportion  than  the  dimi- 
nution in  the  time  of  its  flow.  Thus,  if  the  steam  be 
cut  ofl*  at  half  stroke,  its  tension  will  be  more  than 
doubled.  When  admitted  of  such  increased  tension 
into  the  cylinder,  and  cut  ofl*  after  the  cylinder  is 
partly  filled,  the  steam  will  expand  and  continue  to 
act  by  its  elastic  force ;  and  if  its  final  expansion  do 
not  reduce  its  tension  below  the  measure  of  resistan- 
ces, the  stroke  will  be  completed.  In  this  way,  by 
cutting  off*  the  steam  early  in  the  stroke,  vast  advan- 
tages have  been  gained,  and,  in  some  instances,  the 
effect  of  a  given  engine  and  boiler  has  been  more 
than  quadrupled,  without  any  additional  expenditure 
of  fuel. 

112.  In  order  to  fit  an  engine  for  acting  expan- 
sively, the  boiler  must  be  made  of  such  form  and 
material  as  will  enable  it  safely  to  bear  the  increased 
tension  of  the  steam ;  a  valve,  to  cut  off*  the  steam 
at  the  required  part  of  the  stroke,  muist  be  placed  in 
the  steam  pipe,  and  apparatus  for  working  it  provi- 
ded ;  instead  of  a  common  pump,  the  hot  water 
pump  must  be  capable  of  forcing  the  supply  into  the 
boiler,  and  the  feeding  apparatus  appropriate  to  a 
low  pressure  boiler  removed  .*  Of  all  modes  in  which 
steam  has  yet  been  applied,  this  is  found  to  be  most 
advantageous. 

*  For  a  full  exposition  of  the  advantages  of  a  condensing  en 
gine  acting  expansively,  see  the  author's  "  T'oatiseon  the  Steam 
engine." 


PRACTICAL   MECHANICS.  103 

113.  High  steam  may  be  also  used  without  being 
condensed.  The  engine  is,  in  this  case,  said  to  be 
"high  pressure."  This  method  was  originally  pro- 
posed as  early  as  the  time  of  Watt's  improvement, 
by  OUver  Evans,  an  American  engineer.  It  was 
carried  into  effect  by  him  and  by  Trevithick  in  Eng- 
land at  the  same  time,  in  1801. 

114.  A  high  pressure  engine  may  have  the  same 
general  form  as  a  condensing  one,  being  composed 
of  a  cylinder,  parallel  motion,  working-beam,  con- 
necting-rod, crank,  and  fly-wheel.  The  cold  water 
cistern,  condenser,  and  airpump  are  unnecessary, 
and  are  therefore  suppressed.  For  the  hot  water 
cistern  is  substituted  a  reservoir,  in  which  water  may 
be  heated  by  the  waste  steam  from  the  cylinder ;  this 
is  supplied  by  a  common  pump,  and  the  water  is 
forced  from  it  into  the  boiler  by  a  forcing  pump,  both 
worked  by  the  engine. 

115.  Instead  of  the  conical  valves  which  have 
been  described,  or  the  long  slide  valve,  a  short  slide 
valve  is  usually  substituted.  The  structure  and  use 
of  this  may  be  understood  from  Fig.  36. 

Fig.  36.. 


A.  Steam  chest.  c  c.  Rod  connected  with  the  turn 

B.  Moveable  septum,  which  al-  bling  shaft, 
ternately  covers  the  entran-  d  d.  Side  pipes. 
ce«  of  the  side  pipes. 


104  PRACTICAL    MECHANICS. 

116.  The  cylinders  of  high  pressure  engines  are 
frequently  placed  horizontally.  In  this  form  of  engine 
the  parallel  motion  and  working-beam  are  suppress- 
ed, and  the  connecting-rod  makes  a  communication 
directly  between  the  head  of  the  piston-rod  and  the 
crank. 

117.  The  high  pressure  engine  derives  its  value 
in  practice  from  the  fact  that  the  tension  of  steam 
increases  in  a  higher  ratio  than  its  density,  when  its 
temperature  is  elevated  :  and  thus,  although  it  is  al- 
ways resisted  by  the  pressure  of  the  atmosphere,  a 
point  will  be  reached  at  which  its  duty  will  be  ex- 
actly equal  to  that  of  a  condensing  engine  working 
with  low  steam  not  cut  off.  This  equality  is  attained 
when  the  tension  of  the  steam  employed  is  equal  to 
three  atmospheres.  A  farther  increase  of  tension 
will  increase  the  duty  of  the  engine,  but  in  a  far  less 
ratio  than  in  the  expansive  action  of  the  condensing 
engine. 

118.  High  pressure  engines  are  therefore  inferior 
to  the  condensing  engine  acting  expansively  in  the 
economy  of  the  power.  They  have,  however,  certain 
merits,  which  cause  them  to  be  used  in  many  cases 
w^here  the  condensing  engine  might  be  employed. 
Their  first  cost  is  much  less  ;  they  are  much  simpler, 
and  much  more  readily  kept  in  repair.  In  addition,  j 
they  are  the  only  engines  which  can  be  applied  to  lo- 
comotion, for  in  this  instance  cold  water  for  the  pur- 
pose of  condensation  cannot  be  supplied. 

119.  When  high  steam  is  used,  it  is  possible  to  J 
apply  it  upon  the  same  principle  on  which  water  is  | 
used  in  Barker's  Mill  (see  §  45),  or  by  its  reaction. 
A  very  ingenious  and  efficient  engine  of  this  descrip- 1 
tion  has  been  constructed  in  the  United  States  by] 
Avery      Its  duty  has  been  found,  by  actual  experi- 1 


PRACTICAL   MECHANICS.  105 

merit,  to  be  superior  to  that  of  the  high  pressure  en- 
gine and  the  condensing  engine  acting  with  low 
steam,  but  inferior  to  the  latter  acting  expansively.* 

120.  From  a  mistaken  notion  of  a  loss  of  power 
attending  the  action  of  a  crank,  rotary  engines,  or 
those  in  which  a  continuous  circular  motion  is  pro- 
duced directly  by  the  steam,  have  been  much  sought 
for.  This  notion  is  not  correct ;  and  even  if  a  suc- 
cessful rotary  engine  should  be  constructed,  no  im- 
portant advantage  would  be  derived  from  this  cause. 
There  would,  however,  be  a  greater  degree  of  sim- 
{)licity  in  the  gearing  of  such  an  engine,  which  might 
be  of  use  independently  of  any  other  reason.  While, 
therefore,  the  advantages  which  projectors  have  an- 
ticipated from  rotary  engines  are  not  real,  there  may 
still  be  a  sufficient  gain  to  warrant  an  attempt  at  con- 
structing them. 

*  For  a  more  full  account  of  the  Steam-engine  in  its  most  im 
■^"•oved  forms,  see  the  author's  "  Treatise  on  the  Steam-engine." 


106  PRACTICAL   MECHANICS. 


III. 

MACHINES   MOVED    BY   DESCENDING   WEIGHTS. 

121.  A  WEIGHT  tends  to  descend  with  a  velocity 
uniformly  accelerated.  This  acceleration  may  be 
done  away,  and  the  motion  rendered  uniform  after  a 
time,  by  applying  a  resistance  which  increases  in  a 
higher  ratio  than  the  velocity. 

The  air  furnishes  a  resistance  of  this  sort,  and 
hence  it  has  been  attempted  to  regulate  machines  set 
in  motion  by  a  descending  weight,  by  placing  leaves 
or  thin  plates  of  metal  at  right  angles  to  the  surface 
of  a  horizontal  fly-wheel.  Were  the  density  of  the 
air  constant,  this  mode  of  regulation  would  be  per- 
fect ;  but,  in  consequence  of  the  continual  variations 
in  temperature  and  pressure,  it  ceases  to  be  efficient 
in  those  cases  where  accuracy  is  required. 

This  is  more  particularly  the  case  in  the  clock, 
where  regularity  of  motion  is  the  only  desired  object, 

122.  Failing  in  the  application  of  the  fly  with 
leaves  to  regulate  instruments  intended  for  the  meas- 
ure of  time,  the  pendulum*  is  now  universally  applied 
to  the  purpose. 

A  pendulum  itself,  from  its  near  approach  to 
isochronism,  would  be  a  good  measure  of  equal  por- 
tions of  time,  were  it  not  that  it  loses  a  portion  of  its 
motion  at  each  oscillation,  in  consequence  of  friction 
upon  its  axis  of  suspension,  and  the  resistance  of  the 

*  For  the  theory  of  the  pendulum,  see  the  author's  treatise  on 
Mechanics.     See  also  Mosely's  Illustrations. 


PRACTICAL    MECHANICS.  107 

air.  Its  oscillations,  in  consequence,  are  not  abso- 
lutely equal  in  time,  and  it  finally  ceases  to  oscillate. 
The  force  of  a  descending  weight  may  be  applied  for 
the  purpose  of  restoring  to  the  pendulum  at  each  os- 
cillation the  motion  it  has  lost,  by  means  of  a  train 
u[  wheels  and  pinions.  As  this  loss  is  extremely 
small,  the  intensity  of  the  action  of  the  weight  is  less- 
ened by  causing  wheels  to  drive  pinions.  In  this 
way,  too,  as  the  descent  of  the  weight  will  be  checked 
by  the  pendulum,  reacting  through  pinions  driving 
I  wheels,  this  descent  will  be  slow,  and  the  weight  will 
I  take  a  considerable  time  to  pass  through  a  small 
space.  The  apparatus,  therefore,  may  continue  in 
motion  for  several  days  without  occupying  much 
room.  The  train  of  wheels  and  pinions  may  be  made 
to  subserve  another  purpose,  for  those  which  have 
the  slowest  motion  are  capable  of  registering  upon  a 
divided  circle  the  revolutions  of  those  which  move 
faster  than  themselves,  and  of  subdividing  each  of  their 
own  revolutions, 

123.  A  clock,  then,  is  an  instrument  intended  for 
the  measure  of  time,  and  is  composed  of  a  pendulum, 
adapted  as  a  regulator  to  a  machine  usually  moved 
by  a  weight,  which  machine  counts  and  records  the 
number  of  the  pendulum's  oscillations. 

124.  It  is  difficult  to  estimate  exactly  the  quantity 
of  motion  lost  by  a  pendulum  at  each  oscillation  ;  it 
therefore  becomes  necessary,  in  order  to  prevent  the 
clock  from  stopping,  to  give  it  motion  by  a  weight  of 
more  intensity  tl^an  would  be  barely  sufficient.  Un- 
der the  actifj[|  of  such  a  weight,  a  tendency  to  accel- 
eration will  ensue,  but  this  will  be  counteracted  by 
the  increased  arc  in  which  the  excess  of  force  would 
tend  to  make  the  pendulum  swing,  and  an  increase 
in  the  arc  will  reauire  a  longer  time  for  its  descrip- 


108  PRACTICAL   MECHANICS. 

tion.  Thus,  after  a  time,  the  arc  will  become  constant, 
and  the  oscillations  equal.  A  clock  is  now  said  to 
take  up  its  rate,  and  it  will  do  this  the  sooner,  as  the 
excess  of  the  action  of  the  weight  is  less.  From  this  i 
it  follows  that  a  clock  ought  not  to  be  allowed  to  I 
run  down,  and  should  be  so  constructed  that  the  mo- 
tion may  be  kept  up  while  the  weight  is  in  the  act  of 
being  lifted  back  to  its  primitive  position. 

It  will  appear,  from  what  has  been  stated,  that  the 
less  the  weight  that  will  ensure  the  clock  to  go,  the 
better.  The  weight  is  therefore  never  sufficient  to 
cause  the  pendulum  to  begin  to  move,  but  the  latter 
must  be  raised  through  half  its  arc  of  oscillation  by- 
hand,  and  then  left  to  itself.  The  oscillation  thus 
commenced  by  the  falling  of  the  pendulum  from  the 
position  to  which  it  has  been  raised,  is  afterward 
kept  up  by  the  action  of  the  weight. 

125.  We  have  supposed  the  pendulum  to  be  of 
constant  length  ;  but  this  cannot  be  the  case  when 
the  pendulum  is  composed  of  a  simple  metallic  rod 
bearing  a  bulb,  for  both  are  subject  to  expansion  and 
contraction  by  alternations  of  heat  and  cold.  This 
defect  has  been  obviated  by  various  modifications  of 
the  pendulum,  by  which  it  is  said  to  be  compensated, 
or  caused  to  remain  of  nearly  invariable  length  du- 
ring all  the  changes  of  temperature  which  can  occur 
by  exposure  to  the  climate. 

Two  different  principles  have  governed  the  con- 
struction of  compensation  pendulums : 

(1.)  To  make  the  bulb  and  the  rod  of  such  mate- 
rials, that  the  expansion  of  the  former  upward  shall 
exactly  equal  the  expansion  of  the  latt^f  downward. 

(2.)  To  make  the  rod  of  several  pieces,  acting  in 
opposition  to  each  other,  in  such  manner  that  the 
joint  expansions  of  a  part  of  them  downward  shall  be 
counteracted  by  the  joint  expansions  of  another  part 
and  of  the  bulb  upward. 


PRACTICAL   MECHANICS.  109 

The  pendulum  most  in  use,  which  depends  for  its 
structure  on  the  first  principle,  is  the  mercurial  pen- 
dulum of  Graham.  The  most  familiar  pendulum, 
founded  on  the  second  principle,  is  the  gridiron  pen- 
dulum of  Harrison.* 

A  wooden  rod  thoroughly  seasoned,  and  protect- 
ed by  varnish  from  the  moisture  of  the  atmosphere, 
answers  as  a  measure  of  equal  time  nearly  as  well 
as  the  best  compensation  pendulum. 

126.  The  weight  which  moves  a  clock  is  attached 
to  a  cord.  This  cord  is  wound  around  a  barrel,  or 
is  stretched  by  a  counterpoise  over  a  pulley.  When 
the  weight  is  left  free,  it,  in  descending,  turns  the 
barrel  around  and  uncoils  the  cord.  It  is  necessary, 
in  order  that  the  weight  may  be  wound  up  after  it 
has  finished  its  descent,  without  turning  the  wheels 
back  again  through  the  whole  of  their  respective 
revolutions,  that  this  barrel  should  be  attached  to  the 
first  wheel  of  the  train  by  some  method,  which, 
while  it  conveys  the  whole  impulse  of  the  weight  in 
one  direction,  shall  permit  free  motion  in  the  other. 
This  is  effected  by  means  of  the  ratchet  and  ratchet- 
wheel  ;  the  former  of  which  is  attached  to  the  first 
wheel  of  the  train,  the  latter  firmly  fixed  to  the  bar- 
rel. The  ratchet  and  ratchet-wheel  are  represented 
Fig.  37. 

127.  In  the  common  eight-day  clock  there  are 
four  wheels,  which  are  enclosed  between  two  plates, 
ill  which  their  axles  rest.  The  first  or  great  wheel 
has  the  same  axis  with  the  barrel,  and  is  attached  to 
it  by  means  of  the  ratchet-wheel.  The  great  wheel 
revolves  in  twelve  hours,  and  might  therefore  carry 

*  For  the  theory  of  these  pendulums,  see  the  author's  "  Treat- 
ise on  Mechanics."  For  a  full  account  of  various  compensation 
pendulums,  see  Kater,  in  Lardner's  Cyclopaedia,  article  "  Mechan- 
ics."   See  also  Mosely's  Illustrations. 


110  PRACTICAL   MECHANICS. 

Fig.  37. 


*the  hour-hand,  which  performs  a  circuit  of  the  diai 
plate  in  that  space  of  time ;  it  has  96  teeth,  and 
turns  a  pinion  of  8  teeth.  This  pinion  is  fixed  to 
the  same  axle  with  a  wheel,  which  revolves  with  it 
in  an  hour,  and  is  hence  called  the  hour- wheel,  or, 
from  its  position  opposite  to  the  middle  of  the  dial, 
the  centre  wheel.  The  hour- wheel  has  64  teeth,  and 
turns  a  pinion  of  8  teeth.  This  pinion,  in  the  ori- 
ginal form  of  clocks,  carries  upon  its  axle  a  wheel 
whose  teeth  are  at  right  angles  to  the  plane  in  which 
it  revolves,  and  which  is  hence  called  the  contrate 
wheel.  The  contrate  wheel  has  60  teeth,  and  turns 
a  pinion  of  8  teeth.  The  axis  of  this  pinion  is 
vertical,  the  contrate  wheel  having  the  property  of 
communicating  a  motion  at  right  angles  to  that  in 
which  itself  revolves.  On  the  vertical  axle  of  this 
pinion  is  situated  a  wheel,  called  the  swing  or  crown- 
wheel, which,  of  course,  revolves  in  a  horizontal 
plane.  This  has  30  vertical  teeth,  unequally  incli- 
ned at  their  two  faces  like  the  teeth  of  a  saw.  Over 
the  crown-wheel  lies  a  rod  called  the  verge,  from 


PRACTICAL    MECHANICS. 


Ill 


Avliich  project  downward  two  leaves  or  pallets  that 

ire  not  in  the  same  plane.     These  lie  over  points  of 

he  circumference  of  the  wheel  nearly  opposite  to  each 

3tlier,  and,  as  the  number  of  teeth  in  the  crown-wheel 

-'  even,  they  will  alternately  receive  impulses  from 

Aid  these  impulses  are  in  opposite  directions.     An 

i  Hating  motion  is  thus  given  to  the  verge,  at  the 

3  of  two  vibrations  for  each  tooth  of  the  crown- 

lcI.       The  verge  is  bent  downward  behind  the 

to  of  the  clock,  and  is  again  bent  horizontally  out- 

A.ird ;    the  last  part  is  forked,  and    the    pendulum 

jcing  dropped  between  the  branches,  receives  from 

hem  the  motion  communicated  to  the  verge.     The 

brked  part  is  called  the  crutch. 

The   arrangement   of  crown-wheel,  pallets,  and 
i^erge  is  represented  in  Fig.  38. 
Fig.  38. 


The  great  wheel  being  situated  far  from  the  centre 
of  the  works  of  the  clock,  and  it  being  usually  con- 
sidered expedient  to  make  the  hour  and  minute  hand 
revolve  upon  the  same  axis,  additional  wheels  and 
pinions,  called  the  dial-work,  are  inserted  between 
the  front  plate  of  the  clock  and  the  dial.  Besides,  if 
the  hour-hand  were  carried  by  the  great  wheel,  it 
would  revolve  in  a  direction  opposite  to  that  in  which 
the  minute-hand  revolves.  In  clocks  where  the 
symmetry  of  position  is  considered  of  less  importance 
than  simplicity,  a  wheel  upon  the  same  axis  as  che 


112 


PRACTICAL   MECHANICS. 


great  wheel  is  made  to  turn  another  wheel  of  the 
same  number  of  teeth  with  itself,  and  which  will 
revolve  at  the  same  rate,  but  in  a  contrary  direction, 
and  the  dial- work  is  omitted. 

A  side  view  of  the  common  clock  is  exhibited  in 
Fig.  39 

Fig.  39. 


PRACTICAL    MECHANICS.  113 


W.  Weight. 

g  g.  Pallets. 

B.  Barrel. 

h  h.  Verge. 

c  c.  Ratchet  Wheel. 

i  i  i.  Crutch. 

a  a.  Great  Wheel. 

p  p.  Pendulum,  represented  as 

d  d.  Centre  Wheel. 

suspended  by  a  thread  from 

e  c.  Contrate  Wheel. 

the  rod  o  o. 

//.  Swing,  or  Balance  Wheel. 

The  wheels  k  k,  I  k,  and  n  n,  with  the  pinion  m  n, 
arc  placed  between  the  front  plate  and  the  dial,  for 
the  purpose  of  reducing  the  motion  of  the  wheel  d  d, 
which  revolves  in  an  hour  and  carries  the  minute- 
hand,  to  one  of  a  twelfth  part  of  the  speed,  by  which 
the  hour-hand  may  be  carried.  The  barrel  s,  and 
the  axle  of  the  wheel  k  k,  are  hollow,  in  order  that 
the  axle  of  the  wheel  d  d  may  pass  through  them. 

128.  The  method  of  converting  the  continuous 
jinotion  of  the  wheels  into  the  reciprocating  motion 
of  the  pendulum,  by  a  crown-wheel  and  pallets,  has 
}the  advantage  of  great  simplicity  and  cheapness.  It 
is  not,  however,  favourable  to  accuracy  in  the  divis- 
ion of  time. 

129.  A  very  simple  clock  was  proposed  and  con- 
structed by  Franklin.  This  has  only  three  wheels. 
The  first  of  these  revolves  in  four  hours,  and  has  160 
teeth.  The  pinion  of  the  second  wheel  has  10  teeth, 
and  the  wheel  itself  120  teeth.  The  third  or  bal- 
ance-wheel has  30  teeth,  and  its  pinion  8.  The  dial- 
plate  is  divided  into  four  quadrants,  each  of  which 
is  divided  in  60  parts,  and  the  hours  are  numbered 
from  1  to  12,  upon  a  spiral  within  the  graduated  cir- 
cle ;  the  spiral  makes  three  revolutions  between  the 
centre  and  the  circumference.  One  hand,  therefore, 
serves  to  point  out  both  the  hours  and  the  minutes. 

A  clock  of  the  same  number  of  wheels,  whose  great 
wheel  revolves  in  12  hours,  was  planned  by  Fergu- 
son ;  but,  in  order  to  remove  some  objections  urged 


114 


PRACTICAL    MECHANICS. 


against  that  of  Franklin,  he  loaded  it  with  parts 
which  are  liable  to  greater  ones. 

A  more  beautiful  and  ingenious  modification  of 
the  clock  of  Franklin  has  been  constructed  by  Bre- 
guet,  and  is  represented  in  Fig.  40.  The  great 
wheel  of  this  has  288  teeth,  the  pinion  it  drives  6 
teeth  ;  the  latter  is  upon  the  axle  of  the  balance 
wheel,  which  has  30  teeth. 

Fig.  40. 


PRACTICAL  MECHANICS.         115 

This  clock,  like  many  other  French  timekeepers, 
has  an  arrangement  called  the  equation,  by  which 
the  mean  time  it  marks  is  converted  into  apparent 
time.  This  is  rendered  expedient  by  the  practice  of 
France,  where  the  apparent  time  is  that  by  which 
the  concerns  of  life  are  regulated,  but  would  be  of  no 
value  in  this  country,  where  we  employ  mean  time. 
Tills  equation  apparatus  is  of  much  ingenuity,  but  is 
not  of  sufficient  importance  to  us  to  need  description. 

Clocks,  except  when  intended  for  astronomic  pur- 
poses, have  often  an  additional  set  of  works  for  the 
purpose  of  striking  the  hours,  and  occasionally  the 
half  hours,  or  even  quarters.  This  is  moved  by  a 
weight,  but  is  locked  except  just  at  the  time  it  is  to 
strike,  when  the  other  set  of  works  removes  the  de- 
tent. The  regulator  of  the  motion  is  a  small  fly 
with  leaves. 

130.  Any  mode  by  which  the  continuous  revolu- 
tion of  a  timekeeper  can  be  converted  into  a  recip- 
rocating motion,  is  called  a  scapement.  The  crown- 
wheel and  pallets  do  not  furnish  a  good  scapement, 
(or  several  reasons.  1.  The  pendulum  swings  in 
lOo  large  an  arc,  and  is  hence  more  subject  to  vari- 
ation than  were  the  arc  smaller.  2.  The  pendu- 
lum and  weight  are  never  wholly  free  from  each 
3ther,  and  the  former  is,  therefore,  continually  sub- 
ject to  the  accelerating  influence  of  the  latter.  3. 
:V  recoil  or  reversed  motion  ensues  from  the  mutual 
iction  of  the  weight  and  pendulum. 

These  objections  have  been  obviated  by  various 
3ther  scapements,  by  which  the  arc  has  been  much 
diminished,  the  pendulum  left  free  during  its  entire 
Dscillation,  and  the  descent  of  the  weight  complete- 
ly checked,  but  without  recoil,  at  the  end  of  each  os- 
nllation  of  the  pendulum.  When  scapements  have 
.he  second  of  these  properties,  they  are  said  to  be 


116  PRACTICAL    MECHANICS, 

free  or  detached  ;  when  they  produce  the  last  effect, 
the  clock  is  said  to  make  a  dead  beat.  Of  these 
scapements  the  most  interesting  are  as  follows,  viz.  : 

(1.)  The  anchor  scapement,  as  represented  in  Fig. 
7,  page  21. 

This  is  still  liable  to  recoil. 

(2.)  The  improved  anchor  scapement,  by  which 
recoil  is  obviated.  This  improvement  was  the  in- 
vention of  Graham,  and  a  form  of  it,  applied  by  Reid 
to  a  clock  in  the  collection  of  Columbia  College,  is 
represented  Fig.  41. 

Fig.  41. 


(3.)  The  scapement  of  Amant  or  d  clievilles,  rep- 
•esente^Fig.  42. 

There  are  various  other  scapements,  of  greater 
iomplexity,  which  are  applicable  to  clocks ;  those!  ■ 
which  have  been  described  are,  however,  the  most 
interesting,  and  sufficient  for  most  practical  purposes. 


PRACTICAL   MECHANICS. 

Fig.  42. 


117 


131.  The  best  clocks,  therefore,  in  conformity 
with  what  has  been  stated,  have  one  or  other  of  the 
improved  scapements  which  have  been  described,  a 
compensation  pendulum,  and  an  apparatus  for  con- 
tinuing the  motion  of  the  great  wheel  while  the 
weight  is  in  the  act  of  being  wound  up.  In  order 
to  prevent  wear  and  lessen  the  friction,  the  pallets 
are  often  constructed  of  corundum  or  other  hard 
stone.  In  astronomic  clocks,  which  are  intended 
to  mark  sidereal  time,  the  motion  of  the  hour-hand 
is  taken  off  from  the  great  wheel  in  such  manner  as 
10 


118         PRACTICAL  MECHANICS. 

to  cause  it  to  revolve  in  twenty-four  hours  instead  of 
twelve. 

With  such  improvements  and  great  nicety  of  work, 
manship,  clocks  have  been  constructed  which  have 
not  varied  more  than  a  fraction  of  a  second  from 
their  rate  for  a  whole  year.  The  most  remarkable 
instance  of  this  sort,  is  a  clock  made  by  Gumming, 
the  property  of  Captain  Brown,  of  London,  which 
was  used  by  Kater  and  Sabine  in  their  experiments 
on  the  pendulum. 

1.32.  Much  of  the  perfection  and  cheapness  of  mod- 
ern clocks  depends  upon  the  application  of  the  me- 
chanical principle  of  the  division  of  labour.  The 
manufacture  of  clocks  includes  sixteen  different 
trades,  and  each  of  these  comprises  several  subordi- 
nate departments,  on  each  of  which  separate  work- 
men  are  exclusively  employed. 


PRACTICAL   MECHANICS.  119 


IV. 

OF   MACHINES   MOVED   BY   SPRINGS. 

133.  It  has  been  stated  that  clocks  are  usually 
moved  by  weights.  Springs  may  also  be  employed 
for  the  same  purpose,  and  this  is  usually  the  case  in 
those  of  small  size,  which  are  regulated  by  half-sec- 
ond pendulums.  Springs,  however,  are  more  fre- 
quently used  in  the  species  of  timekeeper  called  a 
watch.  As  these  are  intended  to  be  portable,  they 
can  neither  be  impelled  by  a  descending  weight,  nor 
regulated  by  a  pendulum,  for  the  motion  of  either  of 
these  is  interrupted  by  any  disturbance. 

134.  The  spring  which  acts  as  the  prime  mover  in 
a  watch,  and  is  called  the  mainspring,  is,  as  has  been 
stated  in  §  17,  of  the  figure  of  a  spiral,  and  is  coiled 
in  a  barrel,  to  which  it  gives  motion.  This  barrel, 
in  moving  under  the  elastic  force  of  the  spring,  coils 
a  chain  around  it,  drawing  this  chain  from  another 
barrel  called  the  fusee.  As  the  spring  acts  with  a 
varying  intensity,  this  barrel  is  made  conoidal,  and 
in  withdrawing  the  chain  from  it,  the  spring  begins 
to  act  upon  its  smallest  diameter  ;  but,  as  the  chain 
is  withdrawn,  the  diameter  on  which  it  acts  increases. 
This  is  effected  by  cutting  a  spiral  groove  around  the 
fusee.  The  figure  which  would  meet  the  law  of  the 
spring's  elasticity,  is  one  formed  by  the  revolution  of  a 
hyperbola  around  its  assymtote.  As  the  spring  can- 
not be  absolutely  homogeneous  or  uniform  in  struc- 
ture, this  figure  is  only  an  approach  to  the  truth.  A 
figure  which  will  compensate  the  varying  action  of 


120 


PRACTICAL    MECHANICS. 


any  given  spring,  is  produced  by  causing  the  spring 
itself  to  act  as  the  moving  power  in  an  engine,  by 
which  the  spiral  groove  is  cut.  The  barrel  which 
contains  the  mainspring,  the  fusee,  and  chain,  are 
exhibited  Fig.  43. 


Fig.  43. 


135.  The  fusee  is  connected  with  the  great  wheel 
of  the  watch  by  means  of  a  ratchet,  and  in  the  better 
class  of  watches  a  spring  is  applied  to  it,  by  which 
it  is  kept  in  motion  while  the  mainspring  is  in  the 
act  of  being  wound  up. 

136.  The  watch  has  five  wheels  driving  four  pin- ' 
ions.  The  second  wheel  is  in  the  centre  of  the 
works,  and  derives  its  name  from  this  position.  The 
fourth  wheel  is  a  contrate  wheel,  and  the  fifth  a 
crown-wheel,  called  the  balance-wheel.  The  bal- 
ance-wheel acts  upon  two  pallets  attached  to  a  verge, 
to  which  it  thus  gives  an  oscillating  motion.  In  or- 
der to  effect  the  regulation  of  the  watch,  a  fly-wheel, 
called  the  balance,  is  attached  to  the  verge,  and  the 
oscillations  of  this  are  confined  by  a  small  spiral 
spring,  attached  to  the  verge  at  one  end,  and  to  a  fixed 
point  at  the  other.  This  spring,  from  its  dimen. 
sions,  is  called  the  hairspring. 

The  balance,  hairspring,  verge,  pallets,  and  crown- 
wheel, are  represented  Fig.  44. 

137.  The  rate  of  a  watch  is  originally  adjusted  by 
altering  the  length  of  the  spring  between  the  points 


PRACTICAL   MECHANICS. 

Fig.  44. 


121 


to  which  it  is  fastened.  This  adjustment  may  be 
subsequently  varied  by  altering  the  position  of  the 
point  to  which  its  outer  coil  is  attached.  The  appa- 
ratus for  the  latter  purpose  is  called  the  regulator. 

138.  Were  the  mainspring  and  that  which  con- 
trols the  motion  of  the  balance  of  equal  strength,  the 
one  might  control  the  varying  elasticity  of  the  other. 
But,  as  this  cannot  be  the  case,  it  becomes  neces- 
sary to  compensate  the  rate  of  the  watch  for  the 
change  in  the  elasticity  of  the  springs,  produced  by 
heat  and  cold.  This  was  at  first  attempted  by 
means  of  a  curb  applied  to  the  hairspring,  and  act- 
ing on  the  same  principle  as  the  gridiron  pendulum. 


122  PRACTICAL    MECHANICS. 

The  compensation  is  now  effected  in  the  balance 
itself,  whose  weight,  and  the  distance  of  the  circle  in 
which  its  force  may  be  considered  as  acting,  affects 
the  intensity  with  which  the  hairspring  reacts.  The 
rim  of  the  balance  is  cut  into  segments,  each  of  which 
is  joined  to  a  spoke  at  one  end  only.  The  segments 
are  formed  of  arcs  of  two  different  metals  firmly  uni- 
ted. As  these  metals  have  different  rates  of  expan- 
sion by  heat,  their  unequal  variation  in  length  will 
cause  the  curvature  of  the  arc  to  change,  and  alter 
the  distance  of  the  free  end  of  the  segment  from  the 
axis.  In  order  to  give  the  balance  more  power,  this 
end  of  the  segment  is  loaded  until  the  balance  can 
no  longer  be  set  in  motion  by  the  force  of  the  main- 
spring, transmitted  to  it  through  the  train  of  wheels. 

It  will  be  easily  seen  that  the  varying  curvature  of 
the  segments  might  be  made  to  counteract  the  vary- 
ing length  of  the  spokes,  and  thus  make  the  efficient 
diameter  of  the  wheel  constant.  This  is  not,  howev- 
er, done,  for  the  structure  of  the  balance  must,  in  ad- 
dition, be  made  to  compensate  all  the  different  ac- 
tions of  heat  upon  the  whole  works. 

A  compensation  balance,  by  Arnold,  is  represented 
in  Fig.  45. 

139.  The  crown-wheel  and  pallets  are  equally 
objectionable  as  a  scapement  in  the  watch  as  in  the 
clock.  A  variety  of  others  have,  therefore,  been 
proposed,  in  all  of  which  the  contrate  wheel  is  chan- 
ged into  one  whose  teeth  lie  in  its  own  plane,  and 
thus  the  balance-wheel  is  caused  to  move  parallel  to 
the  others. 

A  scapement  which  has  much  celebrity  is  known 
by  the  name  of  the  patent  lever.  In  this,  the  bal- 
ance-wheel and  pallets  are  such  as  have  been  descri- 
bed in  §  130,  under  the  name  of  the  anchor  pallets. 


PRACTICAL  MECHANICS. 

Fig.  45. 


123 


To  the  fixed  point  of  these  pallets  a  bar  is  attached, 
in  the  direction  of  a  radius  of  the  balance-wheel ;  at 
the  extremity  of  this  bar  or  lever  is  a  segment  of  a 
circle  cut  into  teeth.  These  teeth  catch  into  those 
of  a  pinion  which  surrounds  the  verge. 

The  scapement  most  frequently  used  in  French 
and  Swiss  watches  is  that  d  cylindre,  which  usually 
goes  by  the  name  of  its  inventor,  Lepine.  It  has 
the  form  shown  in  Fig.  46,  on  page  124. 

The  best  scapement  for  a  pocket  watch  is  that  of 
Duplex,  represented  in  Fig.  47,  on  page  124. 

When  the  utmost  accuracy  is  required,  the  chro- 
Qometer  scapement  is  employed.  Of  this  there  are 
various  kinds.     The  most  usual  of  these  is  the  in- 


124 


PRACTICAL    MECHANICS. 


vention  of  Arnold,  of  London,  exhibited  in  Fig,  48 
on  the  opposite  page.  ' 

^  This  scapement  admits  of  a  free  or  detached  mo 
lion  in  the  balance. 


Fig.  46. 


PRACTICAL    MECHANICS.  125 

Fig.  48. 


To  lessen  the  friction,  the  ends  of  the  verge,  and 
sometimes  of  the  wheels  nearest  to  the  scapement  in 
order,  are  ground  to  a  point.  These  points  rest  in 
shallow  cups  of  hard  stone.  In  chronometers  and 
watches  of  the  best  class,  the  part  of  the  verge  which 
is  worn  by  the  scapement  is  also  frequently  made  of 
hard  stone. 

140.  When  the  scapement  is  of  a  good  description 
and  the  balance  heavy,  the  latter  may,  by  the  aid  of 
the  hairspring,  control  the  irregularities  of  the  main- 
spring without  the  aid  of  a  fusee.  In  this  case  the 
great  wheel  is  attached  directly  to  the  barrel  in 
which  the  spring  is  situated,  by  means  of  a  ratchet. 
This  is  the  more  usual  plan  in  the  Lepine  watches 
and  those  of  Breguet.  In  English  watches  and  all 
chronometers,  the  chain  and  fusee  are  retained. 

141.  A  watch  planned  to  keep  the  most  exact  time, 
and  thus  fitted  for  the  purposes  of  nautical  astrono- 

jmy,  is  called  a  chronometer.     This  has  one  of  the 
iScapements  appropriate   to  it,  a  strong   regulating 
Spring  coiled  in  the  form  of  a  helix ;  a  compensation 
11 


126  PRACTICAL    MECHANICS. 

balance  ;  the  ends  of  the  axles  of  the  balance  and  of 
the  more  rapidly  moving  wheels  are  ground  to  points, 
and  their  sockets  bushed  with  jewels.  In  order  to 
counteract  the  irregularities  in  motion  which  arise 
from  the  fact  that  a  given  tooth  on  either  wheel 
presses  with  variable  intensity  upon  the  tooth  of  the 
pinion  with  which  it  is  in  contact,  the  motion  of  the 
scapement  is  often  produced  by  the  direct  action  of 
a  small  spring  called  the  remontoir.  The  use  of: 
the  mainspring  and  the  train  of  wheels  is  therefore! 
limited  to  winding  up  the  remontoir.  Two  suchi 
springs  must  be  used  in  this  case,  in  order  that  one 
shall  be  wound  up  while  the  other  is  in  action. 

142.  The  form  and  arrangement  of  the  parts  of  a 
common  watch  may  be  understood  from  Fig,  49. 
which  is  a  horizontal  plan  of  the  works. 
Fig.  49. 


a.  Wheel  containing  the  spring,  g.  Third  Wheel. 

b.  Chain.  i.  Axle  on  the  end  of  which,  to 

d.  Great  Wheel.  wards/,  are  seen  the  Crown 

e.  Centre  Pinion.  wheel  and  its  Pinion. 

f.  Centre  Wheel.  h.  Contrate  Wheel. 


PRACTICAL  MECHANICS. 


127 


A  section  of  the  same  watch  is  represented  Fig.  50. 
Fig.  50. 


0.  Is  the  barrel  which  contains 

the  spring. 
6.  The  fusee  and  great  wheel. 
c.  Pinion  of  the  centre  wheel. 

f.  Centre  wheel. 

g.  Pinion  of  the  third  wheel. 


h.  Third  wheel. 

i.  Pinion  of  the  contrate  wheel 

k.  Contrate  wheel. 

n.  Crown  wheel. 

p.  Balance. 

y  z.  Dial  work. 


143.  The  construction  of  a  watch  is  more  difficult 
than  that  of  a  clock,  and  the  regulation  by  hairspring 
and  balance  far  less  accurate  in  principle  than  the 
pendulum.  In  spite  of  these  obstacles,  chronome- 
ters have  been  constructed  which  have  varied  little, 
if  any,  more  than  the  best  clocks  from  their  rates. 

144.  The  cheapness  and  excellence,  even  of  com- 
mon watches,  is  owing  to  the  division  of  labour. 
Each  different  part  is  the  object  of  a  separate  trade, 

I  and  each  separate  trade  is  divided  into  branches, 

;  no  two  of  which  are  exercised  by  the  same  individ- 

I  ual.     The  works  made  in  isolated  parts  are  put  to- 

I  gether  between  the  plates,  and  sold  to  the  persons 

1  who  put  their  name  upon  them  as  makers.     Up  to 

this  time,  the  number  of  separate  trades  which  have 

been  concerned  in  the  fabrication  of  the  watch  is 

twelve.     The  maker  afterward  employs  twenty-one 

different  artists  to  finish  the  watch  and  prepare  it  for 

sale,  and  each  of  these  thirty-three  separate  branches 

has  its  subdivisions  of  labour. 


128  PRACTICAL   MECHANICS. 


MACHINES   MOVED   BY   MEN    AND   ANIMALS. 

145.  The  strength  of  man  has  found  so  many  aids 
and  substitutes  in  the  great  natural  mechanical 
agents,  that  there  are  few  compound  engines  to 
which  his  unassisted  labour  is  exclusively  applied. 
In  many  of  those  capable  of  being  worked  by  man. 
whenever  long-continued  exertion  is  demanded,  the 
strength  of  horses  or  other  animals  maybe  more  adJ 
vantageously  applied.  There  are,  however,  some 
instances  where  the  strength  of  man  is  the  only  con. 
venient  mover  under  the  circumstances  to  which  th^ 
machine  is  subjected.  A  few  of  these  engines  wili 
suffice  as  an  illustration  of  our  subject. 

146.  The  Crane  is  a  machine  made  use  of  fo: 
raising  heavy  weights,  and,  at  the  same  time,  chan 
ging  their  position,  referred  to  the  horizontal  plane 
a  short  distance.  The  crane  is  a  compound  mai 
chine,  made  up  of  a  wheel  and  axle  and  a  pulley 
The  accessory  parts  are  the  Arbor  and  the  JibI 
The  arbor  is  a  vertical  shaft,  having  a  complete  rev 
olution  around  its  axis,  except  when  restricted  by  po 
sition,  as,  for  instance,  when  the  crane  is  place< 
against  a  wdU  The  jib  is  a  projecting  frame  at 
tached  to  the  arbor,  and  revolving  with  it.  The  pu| 
ley,  which  may  be  either  single  and  fixed,  or  a  corn 
bination  of  blocks,  is  adapted  to  the  extremity  of  th 
jib.  The  wheel  and  axle  is  sometimes  attached  t 
the  arbor,  and  sometimes  placed  on  a  separate  bsj 
sis.     The  crane  may  be  portable,  or,  if  not  portabW 


PRACTICAL    MECHANICS. 


129 


may  have  no  other  support  than  its  pressure  on  its 
own  base.  In  such  a  case,  the  machine  must  be  so 
constructed  that,  even  when  loaded  with  the  heaviest 
weight  it  is  capable  of  lifting,  the  line  of  direction  of 
the  centre  of  gravity  shall  fall  within  the  base. 

A  crane  of  this  description  is  represented  in  Fig 
51. 

Fig.  51. 


The  wheel  and  axle  takes,  in  the  above  figure,  the 
form  of  a  windlass.  This  may  be  worked  by  men 
taking  hold  of  pins  adapted  to  its  circumference. 
The  wheel  has  also  been  made  hollow,  and  stairs 
formed  upon  its  inner  circumference.  Men  walking 
upon  these  steps  cause  the  wheel  to  turn  by  their 


130  PRACTICAL   MECHANICS. 

weight.  A  better  method  consists  in  forming  the 
steps  on  the  outside  of  the  wheel,  and  adapting  a  plat- 
form at  the  level  of  the  horizontal  diameter  of  thej 
wheel,  whence  men  may  step  upon  its  circumference. 
This  is  one  of  the  most  advantageous  modes  of  ap-l 
plying  human  strength.  A  man  working  upon  it, 
besides  overcoming  the  friction  of  the  engine,  has  ex- 
erted,  for  ten  hours  per  day,  eight  tenths  of  the  assu- 
med  measure  of  the  power  of  man  (see  §  31). 

When  the  crane  is  placed  within  a  building,  the  I 
top  of  the  arbor  may  be  supported  by  a  beam,  or| 
from  one  of  the  walls.     In  this  case  no  precautions! 
for  balancing  its  centre  of  gravity  are  required.     The ! 
jib  may  now  take  the  simple  shape  of  a  gibbet  form-  i 
ed  of  a  horizontal  beam  supported   by  a  diagonal 
brace.     In  this  form  the  pulley,  instead  of  being  fix- 
ed at  the  extremity  of  the  jib,  may  be  adapted  to  a 
small  carriage  moving  on  a  railroad  laid  upon  the 
horizontal  beam.     This  carriage  may  be  moved  to 
and  fro,  by  passing  a  chain  over  a  wheel  adapted  to  i 
it  for  the  purpose  ;  this  wheel  has  another,  cut  into  1 
teeth,  upon  its  axis,  which  catch  into  a  rack  laid  upon  1 
the  beam.  j 

The  power  of  the  engine  may  be  increased,  al-  I 
though,  as  in  all  similar  cases,  at  the  expense  of  ve-  t 
locity,  and,  consequently,  of  time,  by  cutting  the  wheel 
of  the  wheel  and  axle  into  teeth,  and  driving  it  by  a 
pinion.     The  pinion  itself  is  turned  by  a  winch  or 
crank. 

A  crane  with  these  modifications  is  represented 
in  Fig.  52. 

147.  The  Gin  or  triangle  is  also  composed  of  a 
wheel  and  axle,  or  pulley.  Its  accessory  parts  are  a 
tripod  of  wood,  and  a  hook  whence  the  pulley  is  sus- 
pended. The  wheel  and  axle  is  usually  worked, 
like  a  ship's  windlass,  by  handspikes  passed  into  holes 
cut  in  the  axle  for  the  purpose. 


PRilCTlCAL   MECHANICS. 

Fig.  52. 


131 


148.  In  building  walls  of  heavy  stones,  cranes  are 
occasionally  used  ;  in  some  cases  of  this  sort,  an  en- 
gine called  a  Derrick  is  employed.  This  is  also  a 
combination  of  a  wheel  and  axle  with  a  pulley.  In 
some  instances  it  takes  the  same  form  as  the  gin, 
in  others  the  pulley  is  supported  by  a  gallows  frame, 
to  which  the  wheel  and  axle  is  attached.  This  frame 
is  supported  by  two  ropes  or  stays,  by  hauHng  on 
one  of  which  and  slackening  the  other,  a  stone  raised 
parallel  to  the  face  of  a  wall  may  be  laid  on  the  wall 
itself.  A  still  more  perfect  apparatus  of  this  descrip- 
tion, called  the  Boom-derrick,  has  recently  been  intro- 
duced in  this  city.  This  unites  the  advantages  of 
the  derrick  with  those  of  the  crane,  and  will  raise 
and  set  down  its  load  at  any  point  within  the  circle 
of  which  the  boom  is  the  radius. 


132 


PRACTICAL   MECHANICS. 


Fig.  53. 


149.  In  the  Pile-en 
gine,  a  heavy  weight  is 
slowly  raised  to  a  consid- 
erable height,  and  then 
suddenly  discharged  up- 
on the  head  of  a  pointed 
beam  which  is  to  be  driv- 
en into  the  ground.  It 
is  usually  composed  of 
a  wheel  and  axle  combi- 
ned with  a  single  fixed 
pulley.  The  form  em-  ' 
ployed  in  the  hydrau- 
lic works  of  the  city  of 
New-York  is  represent- 
ed in  Fig.  53. 

a  a  a.  Frame. 

b  b.  Cranks  or  winches  to 
which  the  power  is  applied. 

c.  Pinion  on  the  same  axis  with 
the  winches. 

d.  Wheel  driven  by  the  pin- 
ion c. 

e.  Barrel  on  the  same  axis  with 
the  wheel  d. 

ff.  Rope  by  which  the  ram 
and  follower  are  raised. 

g.  Follower,  having  a  weight 
sufficient  to  return  the  rope 
after  the  ram  has  fallen.  It 
contains  a  pair  of  shears, 
which  close  by  their  own 
weight  upon  a  hook  or  sta- 
ple forming  a  part  of  the 
ram. 

h.  Ram. 

i  i.  Blocks  or  cheeks  with  cur 
ved  surfaces,  on  entering 
between  which  the  shears 
are  opened  and  release  the 
ram. 

ft.  Flywheel. 


PRACTICAL   MECHANICS.  133 

The  treadwheel  furnishes  a  better  mode  of  apply- 
ing human  strength  to  this  engine  than  the  winch. 
By  a  direct  comparison  made  by  Captain  Turnbull 
oil  the  Potomac  aqueduct,  six  men  performed  as 
much  work  with  the  former  as  eight  with  the  latter. 
Major  Smith,  U.  S.  Engineers,  has  used  an  engine 
ill  which  the  men  tread  upon  a  vertical  ladder,  which 
is  even  more  advantageous. 

In  great  hydraulic  works,  as  in  establishing  foun- 
dations for  the  piers  of  bridges,  and  for  wet  or  dry 
docks,  the  strength  of  horses  has  been  substituted  in 
driving  piles  for  that  of  man.  This  substitution 
znight  appear  to  be  difficult,  in  consequence  of  the 
risk  to  which  the  horses  would  be  exposed  of  incur- 
ring injury  at  the  moments  of  the  discharge  of  the 
ram  and  the  attachment  of  it  to  the  follower.  The 
return  of  the  follower  without  reversing  the  motion 
of  the  horses,  or  an  acceleration  that  will  injure 
the  machinery,  also  requires  to  be  provided  for. 
These  several  difficulties  have  all  been  obviated  in 
the  pile  engine  of  Vauloue.  This  is  also  remarka- 
ble for  having  been  one  of  the  few  engines  which 
have  come  perfect  from  the  mind  of  the  inventor,  the 
original  machine  having  every  part  necessary  to  its 
performance,  and  having  received  no  improvement 
except  in  workmanship  and  materials  since  it  was 
first  employed  in  the  structure  of  Westminster  Bridge 
in  1732. 

The  ram  and  follower,  with  the  gallows,  pulley, 
and  inclined  cheeks,  are  similar  to  those  of  the  en- 
gine just  described.  The  shears  of  the  follower  are 
of  more  perfect  structure,  and  have  rollers  on  the 
extremity  of  their  arms,  in  order  to  lessen  their  fric- 
tion on  the  cheeks.  The  wheel  and  axle  has  a  ver- 
tical axis,  and  is  represented  in  Fig.  54. 


134 


PRACTICAL  MECHANICS. 


The  action  of  this  engine  will  be  understood  bv 
the  description  of  this  figure. 


Fig.  54. 


o.  Pinion. 

"'  wPiX^^^^^"""  ^^^  """'^  ^^^^  ^^tli  t^e  Pinion.    This  is  of  sach 

ramfs  disct^^T"'''  '^  '''""  '""""'^'^•"^  g-^o-'ly  -hen  the 

nn  J»^  ^^'^f  °V^''  '^''"'"  ^°*^  g''^^t  ^lieel  are  hollow, 
and  a  spindle,//,  passes  freely  through  them.  This 
spindle  IS  pressed  against  the  end  of  a  lever  fs,  bv 
wX'hf  .'"''"'^  "P°"  ^  '"^^^^  ^  /  '■  The-^  fame 
r?  Itfn  h^"f°\''  P,'"  '  ''  ^y^^'"^  '^^  great  wheel 
a  IS  at  ached  to  the  drum  d.  To  the  opposite  end 
of  the  lever/^  cords  are  attached,  which  proceed  to 
the  top  of  the  frame  within  which  the  ram  and  fol- 
lower  move.     When  the  latter  has  reached  its  high. 


PRACTICAL   MECHANICS.  135 

est  position,  and  immediately  after  the  ram  has  been 
discharged,  it  presses  against  these  cords  so  forcibly 
as  to  raise  the  end  of  the  lever  y^,  to  which  they  are 
attached,  and  thus  forces  down  the  other  end  f.  In 
this  way  the  spindle  yy  is  made  to  act  on  the  lever 
ifh,  in  such  manner  as  to  draw  the  pin  i  i  from  the 
drum  d.  The  latter  being  thus  detached,  the  weight 
of  the  follower  is  sufficient  to  turn  it  backward,  and 
the  follower  begins  to  fall.  In  order  to  prevent  the 
acceleration  of  the  descent  of  the  follower,  a  fusee  is 
formed  on  the  top  of  the  drum  d.  Upon  this  fusee  a 
cord  A:  A:  is  coiled  while  the  ram  is  raised.  To  this 
cord  a  small  weight  is  attached,  that  acts  at  dis- 
tances from  the  axis  which  increase  as  the  cord  un- 
coils itself,  and  thus  opposes  the  acceleration  of  the 
follower. 

The  selection  of  machines  we  have  now  made 
will  suffice  as  instances  of  the  manner  in  which 
the  strength  of  men  and  animals  may  be  employed. 
As  respects  the  use  of  the  latter  as  a  prime  mover, 
wheel  carriages  furnish  an  instance  of  more  frequent 
and  general  application.  The  principle  on  which 
the  use  of  these  depends,  and  the  rules  for  construct- 
ing the  roads  on  which  they  move,  are  of  sufficient 
importance  to  entitle  them  to  be  considered  under  a 
separate  head. 


136  PRACTICAL   MECHANICS. 

VI. 

OF   WHEEL   CARRIAGES   AND   ROADS. 

150.  When  the  strength  of  an  animal  is  applied  to 
draught,  the  force  exerted  is  measured  in  terms  of  a 
weight  raised  perpendicularly  upward.  In  the  case 
of  a  horse,  the  most  advantageous  exertion  of  his 
strength  has  been  settled  (§  30)  to  be  that  of  lifting  a 
weight  of  186|d  lbs.  with  a  velocity  of  3^d  feet  per  sec- 
ond for  a  day's  work  of  eight  hours'  duration.  This 
application  of  force  will  move  a  much  greater  weight 
along  a  horizontal  plane,  for  the  resistance  is  no 
longer  the  weight  itself,  but  the  friction  produced  by 
the  pressure  of  the  weight  upon  the  plane  over  which 
it  is  moved. 

151.  This  friction  may  be  diminished  by  causing 
the  body  which  is  to  be  drawn  to  roll  instead  of  sli- 
ding ;  by  laying  the  body  upon  rollers ;  and,  finally, 
by  placing  the  body  upon  a  wheel  carriage.  The 
latter  method  possesses  advantages  in  respect  to 
draught,  growing  out  of  two  causes  : 

(1.)  The  friction  of  a  well-polished  axle  is  no  more 
than  4^oth  of  the  pressure  to  which  it  is  subjected. 

(2.)  The  wheel  is  caused  to  turn  around  by  the  | 
friction  of  its  circumference  upon  the  track.  The  | 
place  where  these  touch  may  therefore  be  taken  as  | 
the  point  of  application  of  the  force,  and  a  mechani- 
cal advantage  is  thus  gained,  upon  the  principle  of 
the  lever,  in  the  ratio  of  the  radius  of  the  wheel  to 
the  radius  of  its  axle. 

If  we  take  for  this  ratio  1-10,  which  is  a  proportion  frequently 
found  in  practice,  the  force  of  a  horse  exerted  in  draught  upon  a 
well-constructed  wheel  carriage  ought  to  be 

186§X40X10=74,666§  lbs. 
or  about  thirty-three  tons. 


PRACTICAL  MECHANICS.  137 

The  actual  advantage  gained  upon  the  best  roads 
j  and  with  the  most  perfect  carriages,  does  not  often 
exceed  one  twentieth  part  of  this,  and  it  is  usual  to 
state  the  force  of  a  horse,  in  drawing  a  carriage  upon 
the  best  level  road,  at  no  more  than  20  cwt.  of  effi- 
cient load,  upon  a  carriage  weighing  10  cwt.  The 
reasons  of  this  vast  discrepance  are  : 

(1.)  The  friction  of  axles,  in  practice  on  a  large 
scale,  is  greater  than  that  inferred  from  experiments 
on  a  small  scale. 

(2.)  Besides  the  friction  upon  the  axle  of  the 
wheel,  a  sliding  friction  takes  place  between  the  hob 
of  the  wheel  and  the  shoulder  of  the  axle  on  the  one 
hand,  or  the  linchpin  on  the  other. 

(3.)  A  friction  takes  place  between  the  faces  of 
the  wheel  and  the  materials  of  the  road. 

(4.)  Inequalities  on  the  road  are  continually  dis- 
turbing  the  regularity  of  the  progressive  motion,  and 
destroying  the  motion  previously  communicated  to 
the  carriage. 

The  effect  of  the  last  cause  will  be  better  under- 
stood when  we  consider  that,  when  the  carriage  is  first 
set  in  motion,  it  must  derive  from  the  prime  mover 
a  quantity  of  motion  which  may  be  estimated  by  the 
product  of  the  measure  of  the  resistance  into  the  ve- 
locity. This  impulse  having  been  once  given,  the 
carriage  would  tend  to  go  on  in  a  straight  line  with 
a  uniform  velocity.  If,  then,  the  motion  on  the  road 
were  truly  in  a  straight  line,  no  other  force  would  be 
necessary  than  as  much  as  would  equal  the  friction. 
But  if,  from  inequalities  in  the  road,  the  velocity 
be  checked  at  irregular  intervals,  and  the  wheels  be 
caused  to  deviate  from  their  proper  course,  the  prime 
mover  must  not  only  overcome  the  friction,  but  re- 
store the  quantity  of  motion  lost  from  this  cause. 

152.  Were  we  to  have  reference  merely  to  the  me- 


138  PRACTICAL  MECHANICS. 

chanical  advantage  which  wheels  possess,  it  might  be 
inferred  that,  with  a  given  diameter  of  axle,  the  great- 
er the  diameter  of  the  wheel  the  better.  This  is  the 
case,  whether  we  consider  their  effect  in  overcoming 
friction,  in  surmounting  obstacles,  or  in  depressing 
them.  If,  however,  the  obstacle  is  to  be  removed, 
the  less  the  diameter  of  the  wheel  the  better ;  but  it 
rarely  happens  that  this  kind  of  action  is  to  be  desired. 
The  height  of  the  wheel  is,  however,  limited  by  the 
direction  of  the  draught. 

A  horse  may  be  considered  as  exerting  his  draught 
very  nearly  in  the  direction  of  his  traces,  or  in  a  Hne 
drawn  from  a  point  in  his  breast  to  the  axis  of  the 
wheel.  When  this  line  is  horizontal,  the  whole  of 
the  force  is  applied  to  draught ;  when  the  draught  is 
directed  upward  from  the  axle,  a  part  of  the  force  of 
the  horse  is  wasted  in  an  attempt  to  lift  a  part  of  the 
weight  of  the  carriage  ;  and  when  the  axle  is  higher 
than  the  horse's  breast,  a  part  of  his  force  is  exerted 
in  producing  a  pressure  upon  the  axle,  by  which  the 
friction  is  increased.  It  is,  however,  said  that,  upon 
a  smooth  and  level  road,  the  mechanical  advantage 
gained  by  an  increased  diameter  of  the  wheel  is  for 
a  time  greater  than  the  4oss  by  the  increased  fric- 
tion ;  and  that  wheels,  whose  axle  is  a  little  higher 
than  the  breast  of  the  horse,  have  been  used  with 
success.  In  all  usual  cases,  an  axle  a  little  lower 
than  the  point  in  the  collar  to  which  the  traces  are 
attached  is  most  advantageous.  This  is  more  par- 
ticularly the  case  on  hilly  roads,  where,  in  rising,  a 
part  of  the  force  must  be  applied  to  overcome  the 
gravity  of  the  load,  and  in  descending,  its  spontane- 
ous velocity  is  to  be  checked.  A  carriage  with  low 
wheels  is  also  less  liable  to  be  upset. 

153.  A  horse,  in  drawing,  leans  forward  upon  his 
collar,  and  would  fall  were  the  resistance  to  be  sud- 


PRACTICAL   MECHANICS.  139 

denly  removed.  He  therefore  acts  not  only  by  his 
muscular  force,  but  also  by  his  weight.  For  this 
reason,  in  drawing  heavy  loads,  horses  of  a  large 
breed  are  found  more  valuable  than  smaller  ones, 
even  of  equal  muscular  strength.  For  agricultural 
purposes,  and  for  transportation  on  turnpike  roads, 
it  is  unwise  to  substitute,  as  has  been  done  in  the 
State  of  New- York,  the  blood  horse  for  that  of  Flan- 
dcrs.  From  this  error  the  Esopus  breed  of  horses, 
one  of  the  most  valuable  known,  has  nearly  become 
extinct.  Blood  horses,  on  the  other  hand,  are  better 
for  travelling  with  speed,  and  are  considered  to  pos- 
sess greater  powers  of  endurance.  They  also  are 
capable  of  recovering  their  strength  after  a  degree 
of  exhaustion  which  would  be  fatal  to  most  other 
races. 

Their  superiority  in  the  latter  respect  over  our 
American  races  is  not,  however,  so  great  as  has  been 
j  generally  imagined.  Pennsylvania  possesses  a  breed 
I  of  horses,  said  to  be  originally  derived  from  that  of 
Esopus,  but  improved  by  crossing  with  those  of  Han- 
over and  Lancashire,  which  sustains  extremes  of  cold 
far  better  than  the  blood  horse,  those  of  heat  equally 
well,  and  which  has  great  powers  of  endurance. 
These  horses,  when  used  upon  the  road,  are  rarely 
sheltered,  and  may  be  seen  sleeping  upon  the  snow 
in  the  streets  of  Philadelphia  in  the  coldest  weather. 
As  a  proof  of  their  powers  of  enduring  fatigue,  we 
have  it  on  the  authority  of  a  general  officer  of  great 
distinction,  that  five  hundred  young  horses  of  this 
breed  were  purchased  for  the  purpose  of  draught  and 
mounting  flying  artillery  at  the  commencement  of 
the  late  war  with  Great  Britain,  and  that,  at  the  end 
of  the  year,  not  a  single  horse  had  been  lost  from  any 
other  cause  than  wounds  received  in  action,  although 
the  service  had  been  one  of  extreme  fatigue. 


140         PRACTICAL  MECHANICS. 

For  the  same  reason  that  the  heavy  horse  is  best 
adapted  to  agricultural  labour,  oxen  have  been  found 
of  great  value  in  all  cases  where  a  small  velocity  is 
sufficient.  This  animal,  however,  has  less  power  of 
recovery  from  fatigue  than  the  horse,  and  this  power 
is  exhausted  after  its  full  growth  has  been  reached. 
It  has  therefore  been  recommended,  that  oxen  be 
worked  no  longer  than  they  continue  to  grow,  and 
be  then  fattened  for  slaughter. 

The  power  of  a  horse  in  draught  is  not  only  aided 
by  his  own  weight,  but  may  be  increased  when  he 
is  loaded  with  an  extrinsic  weight.  For  this  reason, 
a  horse  will  draw  more  in  a  carriage  with  two  than 
in  one  with  four  wheels ;  for,  in  the  former  case,  a 
part  of  the  weight  presses  on  his  back.  It  is  even  a 
practice  with  the  drivers  of  carts,  when  a  horse  meets 
an  obstacle  over  which  he  cannot  force  the  wheel,  to 
mount  upon  his  back,  and  the  obstacle  may  in  this 
case  be  surmounted. 

Upon  this  principle  we  may  explain  a  fact  long  re- 
marked in  the  artillery  service  of  Europe,  namely, 
that  the  horses  which  are  ridden  by  the  drivers  are 
always  in  the  best  order,  and  are  more  capable  of 
enduring  fatigue.  The  same  fact  has  been  remarked 
in  posting  on  the  continent  of  Europe,  where  the 
horse  mounted  by  the  postillion  is  always  found  to 
be  fattest.  This  observation,  however,  is  not  true 
on  the  postroads  of  England,  owing  to  the  difference 
in  the  character  of  the  roads.  In  the  latter  country 
the  roads  are  so  good  that  the  strength  of  the  horse 
is  rather  applied  to  obtain  velocity  than  to  overcome  \ 
friction  ;  and  in  this  case  a  load  is  disadvantageous.  | 

154.  The  advantage  of  carriages  with  two  wheels 
ceases  when  the  weight  is  so  great  as  to  require 
more  than  one  horse  for  its  draught ;  for  it  is  by  no 
means  easy  or  convenient  to   attach  two  or  more  i 


PRACTICAL    MECHANICS.  141 

horses  to  a  two-wheeled  carriage  in  such  a  manner 
that  each  shall  bear  an  equal  part  of  the  weight. 
This  has  indeed  been  attempted  in  the  agricultural 
carriages  of  Flanders,  which  are  often  of  the  form 
of  a  curricle.  Such  carriages,  however,  can  only  be 
employed  in  a  level  country. 

In  order  to  extend  the  advantages  of  two-wheeled 
carriages,  horses  have  been  occasionally  so  trained 
that  a  single  driver  can  manage  five  or  six  carts. 
This  is  done  by  the  farmers  of  Scotland,  who  boast 
much  of  the  superiority  of  this  method,  and  of  the 
good  construction  of  their  carts.  The  cart  used  in 
the  city  of  New- York  is,  however,  fully  equal  to  that 
of  the  Scotch  agriculturists,  and  is  superior  in  most 
respects  to  the  drays  used  in  Philadelphia  and  Balti- 
more, and  to  the  trucks  of  the  eastern  cities. 

Carts  are  also  advantageously  used  when  the 
drivers  can  be  employed  in  loading  them,  as,  for  in- 
stance, in  removing  earth  for  regulating  streets,  and 
in  excavation  and  embankment  for  railroads  and  ca- 
nals. The  facility  with  which  they  are  unloaded 
also  gives  them  great  advantages  in  such  cases. 

155.  In  applying  more  than  a  single  horse  to 
draught,  they  ought  to  be  harnessed  in  pairs,  and  not 
tandem.  In  the  latter  case,  the  wheel-horse  is  ex- 
posed to  much  additional  fatigue.  When  harnessed 
abreast,  no  more  than  two  can  be  conveniently  thus 
placed,  as  the  inner  horses  will  be  distressed  by  the 
pressure  of  the  others. 

In  the  transportation  of  heavy  weights  for  great 
distances,  wagons  drawn  by  four,  six,  or  eight  horses, 
harnessed  in  pairs  and  driven  by  one  person,  are 
most  advantageous. 

156.  In  wagons  and  other  four-wheeled  carriages, 
the  fore- wheels  are  made  of  less  diameter  than  the 

12 


142  PRACTICAL    MECHANICS. 

others,  in  order  that  they  may  pass  beneath  the 
perch  in  turning.  For  all  other  reasons,  such  differ- 
ence is  objectionable.  The  defect  of  the  smaller 
wheels  may  be  remedied,  in  some  degree,  by  loading 
them  with  a  proportion  of  the  weight,  as  much  less 
than  that  borne  by  the  hinder  wheels  as  the  diameter 
of  the  one  is  less  than  that  of  the  other. 

157.  It  is  important,  in  harnessing  horses  abreast, 
that  they  should  be  exactly  matched  in  gait.  Equal- 
ity of  strength  would  also  be  of  great  value,  were  it 
not  that  it  is  possible,  by  placing  the  fulcrum  of  the 
bar  on  which  they  draw,  at  a  distance  from  the  points 
to  which  the  swivel-trees  are  attached,  inversely  pro- 
portioned to  the  respective  strengths  of  the  horses,  to 
bring  them  to  a  condition  of  equality. 

158.  The  spokes  of  wheels  are  not  situated  upon 
a  plane  surface,  but  are  usually  adapted  to  the  hob 
and  fellies  in  such  manner  as  to  lie  in  the  surface  of 
a  cone  of  small  altitude.  Wheels  of  this  figure  are 
said  to  be  dishing.  As  roads  have  an  inclined  or 
curved  section,  this  method  is  often  advantageous,  in- 
asmuch as  the  spoke  on  which  the  greater  part  of  the 
weight  rests  will  be  in  its  position  of  greatest  strength 
when  the  action  upon  it  is  most  intense.  The  axles 
are  also  usually  inclined  downward  from  their  shoul- 
ders. This  construction  was  probably  adopted  for 
the  same  object  as  the  dishing  wheel,  but  may  be 
said  to  be  objectionable  for  every  reason. 

159.  In  carriages  loaded  with  heavy  weights,  the 
face  of  the  wheels  should  be  broad.  Nothing  is  lost 
by  this  construction,  even  upon  hard  and  smooth 
roads,  for  the  friction  does  not  increase  with  the  sur- 
face within  the  limits  to  which  the  breadth  of  wheels 
are  restricted.  In  soft  roads  and  those  cut  into  deep 
ruts,  broad  wheels   possess  a  manifest   advantage. 


PRACTICAL   MECHANICS.  14? 

Carriages  with  broad  wheels  also,  so  for  from  inj 
ring  roads,  tend  to  maintain  them  in  repair. 

In  England,  in  spite  of  the  opposition  of  wagoners, 
the  wheels  of  heavy  wagons  are  required  by  law  to 
have  a  breadth  of  fourteen  inches,  and  this  regula- 
tion has  not  only  lessened  the  cost  of  keeping  roads 
in  repair,  but  is  at  length  admitted  to  have  a  direct 
beneficial  influence  upon  the  profits  of  the  wagoners 
♦h-rmselves. 

160.  Springs,  which  were  originally  introduced 
merely  for  the  ease  of  persons  conveyed  in  carriages, 
are  found  to  give  to  horses  the  power  of  conve}*ing 
heavier  loads  than  they  otherwise  could.  In  produ- 
cing this  effect,  they  permit  the  weight  of  the  load 
supported  by  them  to  act  upon  the  same  principle 
that  a  fly-wheel  does  (§  17). 

161.  The  roads  upon  which  wheel-carriages  travel 
ought  to  be  as  smooth  and  equable  as  possible,  in  or- 
der that  less  effort  may  be  necessarj'  to  continue  the 
motion  at  a  given  velocity.  They  ought  also  to  be 
hard,  in  order  that  the  wheels  may  not  penetrate  and 
form  ruts.  On  the  other  hand,  too  hard  a  material  is 
apt  to  injure  the  feet  of  the  horses,  which  are  rapidly 
destroyed  upon  a  stone  road.  In  spite  of  the  latter 
nbjectiou,  stone  broken  into  fragments  is  the  most  fa- 

irite  material  for  road-making.  In  this  form  it  is 
«.aown  by  the  name  of  metal.  The  stone  used  for 
the  purpose  should  not  be  liable  to  decomposition  by 
the  weather,  nor  to  be  readily  ground  to  powder. 
A  compact  limestone  is,  perhaps,  for  a  combination 
of  advantages,  the  best  road-metal.  The  lai^est 
fragments  of  stone  should  not  exceed  six  ounces  in 
weight,  or  they  may  be  regulated  by  passing  them 
through  a  ring  two  and  a  half  inches  in  diameter. 

A  native  gravel,  whose  fragments  are  not  liable  to 


144         PRACTICAL  MECHANICS. 

decomposition  or  too  rapid  a  wear,  is  the  next  best 
material  for  roads,  and  there  are  cases  in  which  it  is 
.preferable,  particularly  when  it  has  the  property  ot 
binding,  or  uniting  into  a  continuous  mass.  When 
gravel  is  employed,  the  largest  fragments  should  not 
exceed  one  inch  in  diameter. 

These  materials  may  be  laid  upon  any  firm  soil 
after  it  has  been  dressed  to  a  proper  figure.  In  per- 
forming this  operation,  after  removing  the  vegetable 
mould,  the  natural  earth  should  be  as  little  disturbed 
as  possible,  for  earth  recently  moved  is  increased  in 
bulk  about  one  ninth.  This  increase  will  gradually 
be  lost  by  the  settling  of  the  earth,  but  so  much  of  it 
as  is  within  reach  of  the  frost  will  be  disturbed  at 
each  change  of  season.  For  this  reason,  the  use  ot 
the  plough,  which  is  so  favourite  an  instrument  in 
making  and  repairing  the  roads  of  the  United  States, 
is  altogether  objectionable.  If  embankments  are  ne- 
cessary, they  must  be  allowed  to  settle  before  the 
road-metal  is  laid  upon  them.  It  is  better,  except 
where  the  soil  of  the  ground  in  which  the  road  is  to 
be  formed  is  itself  gravel,  to  make  the  cross  section 
of  the  surface  of  the  ground  on  which  the  road  is  to 
be  laid  a  level  line,  and  to  give  the  road  its  proper 
figure  by  the  distribution  of  the  material. 

The  average  thickness  of  the  road-metal  on  a  new 
road  need  not  exceed  six  inches,  for  it  is  better  to 
reserve  any  greater  quantity  for  repairs,  and  partic- 
ularly for  fining  up  ruts  as  often  as  they  are  formed, 
than  to  lay  it  on  at  first.  The  metal  ought  to  be  laid 
on  without  any  attempt  at  arrangement,  in  order  that 
the  pieces  of  diflferent  sizes  may  be  mixed  indiscrim- 
inately. In  this  country  it  has  been  usual  to  put 
upon  roads  as  much  as  three  or  four  feet  of  broken 
stone ;  but  this  involves  a  useless  expense.  It  ha« 
also  been  usual  to  admit  of  stones  of  large  size,  and, 


PRACTICAL    MECHANICS.  145 

in  order  to  remedy  the  defects  of  such  masses,  to 
cover  them  with  gravel  and  sand.  In  cases  where 
gravel  alone  has  been  used,  it  has  been  carefully- 
sorted,  so  that  the  layers  should  be  composed  of  peb- 
bles decreasing  in  size  from  the  foundation  upward. 
This  method  not  only  involves  a  large  expense,  but 
is  vicious,  inasmuch  as  the  heaving  of  the  road  when 
wet,  and  particularly  in  alternations  of  frost  and 
thaw,  will  infallibly  bring  the  largest  pebbles  or 
fragments  of  stone  to  the  surface.  This  rule,  how- 
ever, does  not  apply  where  the  larger  fragments  are 
united  by  mutual  pressure  into  the  form  of  a  pave- 
ment. 

Upon  soft  ground,  as  in  meadows  or  rich  alluvial 
soils,  paved  roads  are  the  cheapest,  and  they  are  in  all 
cases  the  most  durable.  They,  therefore,  are  univer- 
sally used  in  cities  ;  and  upon  the  Continent  of  Eu- 
rope, nearly  all  the  great  roads  are  paved.  The  best 
of  all  roads  are  probably  those  constructed  in  England 
upon  the  plan  of  Telford.  These  roads  have  for  their 
basis  a  pavement  of  rolled  pebbles  laid  in  coarse  sand. 
The  larger  axes  of  these  pebbles  are  placed  upright, 
and  if  either  end  be  more  pointed  than  the  other,  the 
sharper  end  is  set  uppermost.  The  stones  of  least 
dimensions  are  laid  near  the  sides  of  the  road,  so  that 
a  convex  form,  where  required,  may  be  given  by  the 
prominent  points.  The  intervals  between  the  stones 
are  filled  up  with  road-metal,  and  the  whole  covered 
with  that  material  or  gravel  to  the  depth  of  four 
inches. 

Of  the  different  rocks,  those  are  to  be  chosen  for 
road-metal  which  are  not  liable  to  decomposition  or 
desquamation.  Compact  limestone  always  possesses 
this  property,  and  is  among  the  best  of  materials. 
Some  of  the  greenstones  and  granites,  as  well  as 
gneiss,  are  also  well  adapted  to  the  purpose.     Of  the 


146  PRACTICAL    MECHANICS. 

rocks  within  our  own  reach,  a  graywacke,  which  is 
abundant  on  the  banks  of  the  Hudson  below  Albany, 
is  an  excellent  material ;  the  greenstone  of  the  outer 
ridge  of  the  Palisades,  which  begins  at  Weehawken, 
is  too  liable  to  decomposition,  as  is  the  Jersey  sand- 
stone ;  but  the  greenstone  of  Bergen  and  Newark 
mountain  is  desirable.  The  mica  slate  of  our  island 
is  very  easily  decomposed,  and  too  soft.  Of  pebbles, 
an  excellent  variety,  principally  quartz,  is  found  in 
Monmouth  county,  New-Jersey,  where,  although  ob- 
tained from  a  decomposing  conglomerate,  it  is  known 
as  sea-beach  gravel. 

162.  The  least  breadth  of  the  carriage-way  of  a 
road  should  admit  of  the  passage  of  two  carriages,  or 
be  15  feet.  It  may  not  be  necessary  to  pave,  or  form 
of  metal,  more  than  half  of  this  breadth,  except  at  in- 
tervals of  about  100  feet,  where  the  whole  breadth 
should  be  properly  laid,  in  order  that  carriages  may 
pass  each  other  when  the  road  is  affected  by  thaw 
and  frost.  On  much-frequented  roads,  the  breadth 
should  be  30  feet,  so  as  to  admit  four  carriages 
abreast ;  no  more  than  half  of  this,  say  the  middle 
portion,  need  be  constructed  on  Telford's  plan,  or 
even  laid  with  metal,  except  in  the  immediate  vicin- 
ity of  large  cities ;  and  in  the  latter  case  the  road 
ought  to  be  45  feet  in  width. 

Besides  the  carriage-way  and  ditches,  all  roads 
ought  to  have  a  path  on  one,  if  not  on  both  sides, 
for  foot-passengers ;  this  should  be  laid  with  gravel. 
It  is  to  be  regretted  that  this  rule,  so  important  for 
the  public  convenience,  is  wholly  neglected  in  this 
country. 

163.  The  cross  section  of  a  road  must,  on  the 
one  hand,  be  of  such  a  character  as  will  permit 
the  surface  water  to  run  over  it  to  a  ditch  on  the 


PRACTICAL    MECHANICS.  147 

{|ade.  On  the  other  hand,  this  slope  must  not  be 
steep  in  any  part,  otherwise  carriages  may  be  exposed 
to  the  danger  of  upsetting,  and  the  lateral  friction, 
whether  of  the  fellies  or  of  the  axles,  will  be  much 
increased.  On  roads  where  there  are  gentle  slopes 
of  small  inclination,  the  cross  section  of  the  surface 
might  be  a  level  line,  were  it  not  that  the  road  would 
be  liable  to  be  washed.  A  road  is  also  liable  to  in- 
jury from  the  latter  cause,  if  the  curve  of  its  cross 
section  rise  too  rapidly  from  the  vicinity  of  the  ditch. 

It  is  usual  to  make  the  cross  section  of  roads  a 
convex  curve,  not  differing  much  from  a  circular  arc ; 
but  this  figure  is  disadvantageous  in  almost  any 
case,  and  is  in  some  wholly  inadmissible. 

164.  There  are  three  principal  cases  in  the  deter- 
mination of  the  figure  to  be  given  to  the  cross  sec- 
tion of  roads  : 

(1.)  When  a  road  nearly  level  lies  on  ground 
which  is  level  in  the  direction  of  the  cross  section. 
In  this  case  a  ditch  is  to  be  formed  on  each  side  of 
the  carriage-way,  whence  the  metal  should  be  laid  so 
as  to  form  two  planes  of  equal  inclination,  rising 
from  the  ditches  towards  the  middle  of  the  road, 
where  the  two  planes  are  to  be  united  by  a  gentle 
curve. 

The  inclination  of  these  planes  must  not  exceed 
1  foot  in  20,  or  cause  a  difference  of  level  between 
the  crown  and  the  edge  of  the  ditch  of  more  than  9 
inches  in  a  road  of  30  feet  in  breadth.  In  narrower 
roads,  the  inclination  is  to  be  the  same,  and  the  dif- 
ference in  level  diminished  in  proportion  to  the  di- 
minished breadth. 

(2.)  On  side-lying  ground,  the  cross  section  of  the 
road  should  be  a  line  of  uniform  inclination  of  not 
more  than  1  foot  in  20  from  the  outer  edge  of  the 
road  towards  the  higher  ground ;  and  there  should 


148  PRACTICAL    MECHANICS. 

be  but  one  ditch,  lying  between  the  road  and  the  rise 
of  the  hill. 

(3.)  In  a  hollow-way  the  road  must  be  inclined  in 
the  direction  of  its  length,  and  it  is  generally  better 
to  have  but  a  single  ditch,  formed  in  the  middle  of 
the  road,  towards  which  it  is  inclined  equally  on  each 
side. 

Where  a  road  lies  between  two  ditches,  as  in  the 
first  of  the  three  cases,  the  two  inclined  surfaces,  even 
if  permitted  to  meet  at  an  edge,  will  be  speedily  uni- 
ted, by  the  action  of  the  carriages  which  travel  it,  by 
a  continuous  curve.  But  roads  having  for  their 
transverse  section  a  convex  curve  are  in  all  cases 
objectionable,  and  are  to  be  absolutely  excluded  on 
side-lying  ground  and  in  hollow-ways.  In  our  coun- 
try, from  a  misapprehension  of  the  meaning  of  the 
word  turnpike,  no  other  mode  of  forming  roads  has 
been  usually  practised  than  to  heap  them  up  in  the 
middle  by  the  plough  and  scraper.  The  convex 
curve,  thus  formed  is  produced  with  increasing  de- 
clivity, to  the  very  bottom  of  the  ditch. 

Such  roads  have  too  little  inclination  at  the  crown, 
where  water  lodges  and  ruts  are  formed.  They 
compel  the  carriages  to  confine  themselves  to  the 
ridge,  by  which  the  wear  is  restricted  to  a  narrow 
track  ;  they  cause  danger  to  loaded  carriages  at- 
tempting to  pass  each  other,  except  on  the  widest 
roads,  and,  by  the  increase  of  the  lateral  friction,  ma- 
terially diminish  the  loads  which  can  be  drawn  by  a 
given  force.  In  some  of  the  roads  in  the  United 
States,  although  wide  enough  for  four  carriages,  it  is 
unsafe  to  attempt  to  pass  one  which  occupies  the 
crown  of  the  road. 

The  scraper,  by  bringing  to  the  summit  of  the  road 
the  matter  which  has  been  washed  into  the  ditches, 
replaces  the  worst  part  of  the  material  in  a  deterio- 


PRACTICAL  MECHANICS.  149 

ated  state,  and  it  may  be  questioned  whether  it  be 
)ossible  that  we  shall  ever  have  good  roads  in  this 
country  until  this  instrument  be  abandoned.  In. 
itcixd  of  employing  it  to  replace  the  soft  matter  from 
he  ditches,  that  ought  to  be  carefully  removed  from 
he  road  altogether,  the  wear  of  the  road  should  be 
•eplaced  by  fresh  metal,  and  the  ruts  filled  up  with 
ho  same.  A  single  cart  and  driver,  continually  em- 
)loyed  in  carting  gravel,  will  keep  miles  of  country 
;oad  in  good  order. 

165.  It  is  an  excellent  plan  to  pave  the  ditches  of 
roads,  and  particularly  with  flag  stones  when  they 
:an  be  procured.  The  water  which  collects  in  these 
ditches  must  be  carried  off  from  time  to  time  by  cuts 
through  the  footpath  or  bank  of  the  ditch,  and  in 
some  cases  it  becomes  necessary  to  pass  this  water 
beneath  the  road  by  means  of  culverts.  These  are 
always  necessary  upon  side-lying  ground,  where  it  is 
particularly  important  that  no  large  stream  of  water 
shall  form,  and  where  the  water  is  thrown  by  the 
cross  slope  of  the  road  towards  the  rise  of  the  hill,  a 
position  whence  it  cannot  escape  except  by  a  culvert, 
or  by  forming  a  rapid  stream  upon  the  surface  of  the 
road. 

166.  The  best  of  all  roads  for  rapid  summer  trav- 
elling is  composed  of  a  native  gravel  of  such  char- 
acter as  to  bind,  although  this  substance  is  far  from 
durable  under  the  action  of  heavy  carriages.  This 
may  be  known  in  its  own  beds  by  the  difficulty  with 
which  it  is  excavated.  Such  gravel  can  be  found  in 
almost  all  countries,  except  those  which  are  wholly 
composed  of  soft  alluvium.  It  also  forms  the  best 
basis  for  a  paved  road.  Whenever  it  can  be  ob- 
tained  conveniently,  the  loose  superficial  matter  may 
be  removed  to  a  depth  of  six  inches,  and  replaced 


150  PRACTICAL  MECHANICS. 

by  a  bed  of  such  gravel  of  the  thickness  of  a  foci 
Upon  the  middle  of  this  to  the  breadth  of  15  feet  i; 
a  great  road,  or  of  H  in  a  by-road,  a  double  or  sin 
gle  way  of  broken  stone,  or  of  pavement  covered 
with  broken  stone,  is  to  be  constructed,  and  the  slop* 
each  way,  from  this  to  the  ditch  left  in  the  excava 
tion,  completed  with  the  gravel.  In  this  way  a  goo( 
summer,  and  also  a  good  winter  road  will  be  united 
When  the  soil  is  a  native  gravel,  the  construction  o 
the  road  may  be  effected  by  the  plough  and  scraper : 
and  this  is  the  only  case  in  which  they  should  be 
used,  except  for  removing  the  vegetable  mould,  anc 
for  levelling  where  embankments  are  indispensableJ 
These  implements  are  of  great  value,  but  may  be 
productive  of  injury  in  the  hands  of  ignorant  per- 
sons,  who  disturb  unnecessarily  the  foundation  of  the 
road. 

Except  when  the  basis  of  the  road  is  a  pavement, 
a  heavy  roller  may  be  used  to  great  advantage  in 
compressing  the  materials  and  causing  them  to  bind. 

167.  The  slope  of  roads  in  the  longitudinal  direc- 
tion depends  in  some  degree  upon  the  nature  of  the 
country,  but  it  is  generally  possible,  by  a  small  in- 
crease in  their  measured  length,  to  obtain  such  a 
slope  as  may  be  most  advantageous  for  travelling 
upon  them.  This  slope  may  be  best  determined  by 
ascertaining  at  which  incHnation  a  carriage  will  be 
supported  upon  the  road  by  friction.  Experiments 
make  the  friction  vary  between  ^'^-th  and  -j'^th  of  the 
weight;  and  on  well-laid  pavements,  such  as  we 
shall  hereafter  speak  of,  it  has  been  reduced  as  low 
as  e^th.  It  will,  in  all  ordinary  cases,  be  sufficient 
for  practical  purposes  to  take  the  friction  at  4^^^^  ^^ 
the  weight ;  and  if  a  road  be  laid  out  with  a  slope  of 
1  foot  in  40,  or  making  an  angle  of  l^'^  with  the  ho- 
rizon, it  will  be  under  advantageous  circumstances, 


PRACTICAL   MECHANICS.  151 

for  there  will  be  little  need  of  diminishing  the  load  a 
horse  is  employed  to  draw  upon  it ;  and  he  may,  if 
applied  to  obtain  speed,  move  with  equal  freedom  in 
either  direction. 

Any  greater  inclination  is  to  be  avoided ;  for  the 
force  of  the  horse  must,  at  such  inclination,  be  ap- 
plied  not  only  to  overcome  the  friction,  but  to  lift  a 
part  of  the  weight.  The  strength  of  a  horse,  also, 
is  much  diminished  by  moving  on  any  other  surface 
than  a  plane  nearly  level ;  and  at  an  inclination  of 
45^  he  ceases  to  be  able  to  move  even  his  own  weight 
upward,  and  falls  in  descending.  In  reducing  the 
slopes  of  roads  to  the  above  limit  of  4V^^>  i^  is  usual- 
ly more  economic  to  do  it  by  making  the  road  curve 
so  as  to  adapt  itself  to  the  ground,  than  to  attempt 
to  make  it  straight  by  cutting  and  filling.  As  a  gen- 
eral rule,  then,  a  straight  line  is  to  be  avoided  for  a 
road,  except  in  a  country  absolutely  level,  or  with  a 
uniform  slope  in  one  direction,  neither  of  which  usu- 
ally occur  in  practice.  By  making  a  road  curve  in 
such  a  manner  as  to  secure  the  requisite  slope,  the 
effective  length  need  seldom  be  much  increased,  nay, 
may  often  be  lessened  ;  for  the  true  distance  by  a 
road  is  not  its  length  as  it  would  be  projected  on  a 
map,  or  measured  in  horizontal  lines  through  the  air, 
but  will  be  determined  by  the  number  of  turns  a 
wheel  must  make  in  passing  over  it.  A  road  which 
is  continually  bending  on  each  side  of  its  mean  di- 
rection, in  order  to  pass  nearly  level  over  an  undula- 
ting surface,  is  not  only  the  easiest  for  rapid  travel- 
ling and  for  the  transportation  of  heavy  loads,  but  is 
far  more  agreeable  to  the  traveller  from  the  variety 
of  scenery  it  furnishes.  The  practice  of  the  earlier 
settlers  in  our  country  led  to  the  choice  of  such  loca- 
tions for  their  roads,  and  it  was  an  unlucky  circum- 
stance when  the  belief  that  a  straight  road  must  be 


152  PRACTICAL   MECHANICS. 

the  shortest,  led  to  the  abandoning  of  many  of  the  an- 
cient routes.  Our  turnpikes  are,  in  general,  ill  laid 
out,  from  its  having  been  adopted  as  a  principle  that, 
a  road  should  be  straight  provided  the  slopes  upon  it! 
did  not  exceed  5°.  This  rule  was  borrowed  from 
the  extreme  slope  allowed  in  the  road  of  the  Simplon, 
and  thus  it  has  happened  that  many  roads  in  districts, 
whose  mean  surface  is  level,  and  in  which  a  level; 
road  might  have  been  laid  out,  are  continually  rising 
and  falling,  at  an  angle  which  was  only  tolerated  in 
crossing  a  range  of  mountains  of  more  than  double 
the  height  of  any  in  the  United  States. 

To  compare  together  straight  and  curved  roads  in 
an  undulating  countiy,  we  shall  assume  a  case.  If 
the  geographical  distance  between  two  points  in  a  di- 
rect line  be  twelve  miles,  and  if  a  straight  road  be- 
tween them  can  only  be  constructed  by  means  of 
slopes  of  5°,  while  a  level  road  may  be  made  by  in- 
creasing the  distance  to  thirteen  miles,  a  wheel  will 
make  the  same  number  of  turns  on  both  roads,  and 
their  efficient  length  will  be  actually  the  same.  But 
they  are  far  from  equality  in  other  respects.  On  the 
level  road,  a  horse  before  a  light  carriage  may  trot 
the  whole  distance,  while  on  the  inclined  road  he 
must  walk,  unless  the  dangerous  practice  be  adopted 
of  galloping  down  the  hills.  On  the  level  road  the 
horse  will  draw  the  maximum  load,  say  35  cwt., 
while  on  the  inclined  road  he  will  not  draw  more 
than  half  that  weight.  On  the  other  hand,  although 
a  road  may  be  curved  with  advantage,  it  must  not 
ch9,nge  its  direction  suddenly,  and  no  curve  should 
be  admitted  of  a  less  radius  than  100  feet. 

168.  In  conformity  with  the  foregoing  principles,  i 
the  following  rules  may  be  adopted  for  laying  out 
new  lines  of  road  : 

(1.)  Between  two  points  of  nearly  the  same  level 


PRACTICAL   MECHANICS.  153 

and  in  an  undulating  country,  a  route  is  to  be  sought, 
I  which,  if  not  actually  level,  shall  admit  of  no  slope 
I  greater  than  ^^^th ;  and  it  will  be  more  economic  to 
I  do  this  by  curving  the  road  upon  the  natural  surface, 
than  to  attempt  to  level  it  by  excavation  and  em- 
bankment for  the  purpose  of  obtaining  a  straight  road. 

(2.)  Between  two  points  at  different  levels,  a  route 
is  to  be  sought  which  will  give  a  uniform  slope  from 
one  point  to  the  other ;  or,  in  other  words,  the  road, 
when  developed,  should  be  a  plane  of  constant  incli- 
nation. This  inclination,  except  in  extreme  cases, 
as  in  ascending  or  descending  abrupt  and  continuous 
ranges  of  mountains,  must  not  exceed  4Vth. 

(3.)  When  a  ridge  intervenes  between  the  two 
points,  the  lowest  accessible  gap  or  break  in  the 
ridge  is  to  be  sought,  and  the  road  must  be  laid  out 
from  it  in  both  directions  according  to  the  foregoing 
rule. 

It  will  be  easily  seen,  that  many  of  the  turnpike 
roads  in  the  United  States  sin  against  the  foregoing 
rules ;  they  are,  besides,  defective  from  a  badly  cho- 
sen figure  for  the  transverse  section,  and  from  the 
large  size  of  part  of  the  material  which  has  been 
used.  Although  the  latter  may  have  originally  been 
placed  beneath,  it  has  in  all  cases  made  its  appear- 
ance at  the  surface.  The  narrow  wheels  which  are 
permitted  in  carriages  carrying  heavy  weights,  are 
also  continually  cutting  our  most  frequented  roads 
into  deep  ruts,  and  the  prejudices  of  wagoners  seem 
to  preclude  any  hope  of  excluding  this  cause  of  con- 
tinual destruction. 

Taking  all  things  into  account,  we  are  compelled 
to  admit,  that,  with  very  few  exceptions,  the  roads  of 
the  United  States,  when  considered  in  respect  to 
their  cost,  the  general  facilities  of  obtaining  good  ma- 
terials, and  the  small  elevation  of  much  of  our  conti- 


154  PRACTICAL    MECHANICS. 

nent,  are  the  worst  in  the  world.  From  this  gener- 
al rule  there  are,  no  doubt,  exceptions,  and  these  are  ) 
more  usually  in  the  most  difficult  positions,  where  ^ 
native  genius  has  thrown  off  the  false  rules  which  | 
had  been  imposed.  Some  of  our  road-makers,  as,  for  ; 
instance,  the  person  who  originally  laid  out  the  na-  | 
tional  road  from  Cumberland,  seem  to  have  been  of  I 
opinion  that  5°  was  not  a  maximum,  admissible  only  j 
in  an  extreme  case,  but  that  such  a  slope  was  to  be 
sought  in  all  cases. 

A  most  mistaken  notion  has  been  entertained  by 
road-makers,  namely,  that  a  horse,  by  exercising 
different  muscles  in  rising  and  descending,  travels 
with  less  fatigue  upon  an  undulating  road.  This  is 
denied  by  physiologists  ;  and,  even  were  it  true,  the 
diminution  of  load  or  of  speed  would  far  more  than 
compensate  any  gain  in  the  number  of  hours  the  an- 
imal could  work  per  day. 

Among  the  best  specimens  of  laying  out  roads 
which  we  have  seen,  are  :  1st.  One  in  Scotland,  be- 
tween Dumfries  and  Castle  Stuart.  The  geograph- 
ical  distance  between  these  places  is  sixteen  miles, 
and  the  road  measures  twenty  miles.  By  this  in- 
crease the  slopes  have  been  reduced  to  1  foot  in  100, 
with  the  exception  of  two  at  the  summit  of  the  ridge, 
where  they  do  not  exceed  1  foot  in  40.  2d.  One  in 
Putnam  county.  New- York,  leading  eastward  from 
Cold  Spring.  3d.  The  road  laid  out  by  Mr.  Crozet, 
from  Winchester,  Va.,  to  the  South  Branch  of  the 
Potomac.  4th.  The  new  line  of  the  national  road, 
from  Cumberland  westward. 

169.  Pavements  are  not  only  advantageous  as  a 
basis  for  roads,  but  are  absolutely  necessary  in  the 
streets  of  cities.  Any  other  earthy  material  wears 
too  rapidly  to  bear  the  continued  traffic,  gives  rise 
to  much  dust,  and  becoming  mixed  with  filth,  pre- 


PRACTICAL  MECHANICS.         155 

vrents  the  streets  from  being  properly  cleansed.  Such 
pavements  are,  in  our  country,  made  of  rolled  peb- 
bles. These  have  the  advantage  of  forming  a  dura- 
ble road,  and  one  which  may  be  easily  kept  clean. 
It  is,  however,  disagreeable  in  the  motion  it  gives  to 
carriages,  and  is  not  as  easy  to  draught  as  a  well- 
made  gravel  or  Macadamized  road. 

The  pavements  of  London  and  Paris  are  made  of 
:ubical  or  rectangular  blocks  of  hammered  stone  ;  in 
Lh(3  first  city,  of  granite,  in  the  second,  of  the  silicious 
limestone  of  Fontainebleau.  These  are  laid  in  cour- 
ses across  the  street,  and  so  as  to  break  joint  in  the 
direction  of  its  length.  Such  pavements  also  cause 
a  disagreeable  motion  in  carriages,  and  continually 
check  their  motion  at  the  joints. 

In  the  streets  of  Pisa,  in  Italy,  the  best  form  of 
stone  pavement  which  has  yet  been  planned  was  ori- 
ginally introduced.  The  street  is  divided  into  spaces 
corresponding  to  the  breadth  of  a  carriage.  In  each 
of  these,  ranges  of  blocks  of  stone  are  laid  length- 
wise, at  distances  fitted  to  receive  the  wheels  of  a 
carriage.  The  space  between  them  is  filled  with  a 
pavement  of  rounded  pebbles.  Thus  the  horse  has  a 
firm  foothold,  while  the  resistances  are  materially 
diminished.  This  method  has  been  imitated  in  Mi- 
lan and  other  Italian  cities  ;  it  has  also  been  copied 
on  the  RadclifTe  Highway,  London. 

Pavements  are  usually  laid  in  coarse  sand,  upon  a 
bed  of  gravel.  It  would,  however,  be  better  to  lay 
them  in  water  cement,  by  the  aid  of  which  they 
would  become  almost  everlasting.  Such  was  the 
method  used  by  the  Romans  in  their  great  military 
roads.  Of  these,  parts  of  the  Appian  Way  remain 
almost  perfect,  after  a  lapse  of  more  than  twenty 
centuries.  This  road  has  for  its  basis  a  bed  of  ce- 
ment mixed  with  chip  stone.     This  is  covered  with 


156  PRACTICAL    MECHANICS. 

a  second  bed  of  cement,  mixed  with  small  pebbles, 
which  admits  the  stones  to  be  bedded  until  their  up- 
per surfaces  are  of  the  same  height,  and  yet  yields 
them  a  firm  support.  The  stones  are  polygonal 
masses,  obtained  from  the  columnar  rocks  of  neigh- 
bouring volcanic  formations,  and  are  chosen  in  such 
manner  as  to  fit  each  other  as  closely  as  possible. 

170.  A  wooden  pavement,  formed  of  blocks  of 
wood  of  the  shape  of  a  six-sided  prism,  has  been 
used  for  ages  in  Russia,  and  has  recently  been  intro- 
duced, in  an  improved  form,  in  some  of  the  streets  of 
the  city  of  New-York.  These  blocks  are  laid  in 
such  manner  that  the  wear  takes  place  on  the  end 
grain  of  the  timber,  and,  so  far  as  wear  by  carriages, 
ease  of  transportation,  and  cleanliness  are  concern- 
ed, has  given  universal  satisfaction. 

The  last-mentioned  method  might  be  practised 
in  those  parts  of  the  United  States  which  are  still 
covered  with  forests  to  great  advantage.  In  these 
districts,  roads  formed  of  logs  laid  across  the  road 
are  much  used ;  and,  where  sawmills  can  be  erect- 
ed, the  preparation  of  blocks  for  the  purpose  could 
not  be  attended  with  great  expense.  Roads  com- 
posed of  round  logs,  although  rendered  necessary  in 
the  soft  soils  which  are  frequent  in  forests,  are  the 
most  disagreeable  of  any  to  the  traveller,  and  do  not 
admit  of  horses  drawing  a  full  load.  The  objection 
to  the  pavement  with  wooden  blocks  is  its  compara- 
tive want  of  durability.  The  timber  is  under  cir- 
cumstances the  least  favourable  to  its  preservation, 
and  it  has,  in  consequence,  been  found  necessary,  in 
the  experiments  which  have  been  made  in  New- 
York,  to  replace  many  of  the  blocks  annually.  This 
objection  may,  however,  be  obviated,  by  saturating 
the  wood  by  a  process  recently  invented,  which 
promises  to  render  the  duration  of  wood  indefinite. 


PRACTICAL    MECHANICS.  157 

The  substance  with  which  the  wood  is  saturated  in 
this  process  is  coal-tar,  or  native  bitumen. 

171.  A  material  called  asphalte  or  asphaltum,  but 
ich  is  properly  a  peculiar  kind  of  bituminous  lime- 
Lie,  has  recently  been  introduced  in  France.     This 
substance  is  reduced  to  powder,  and  mixed  with  a 
fused   bituminous   substance.     The   liquid   mixture 
may  be  mingled  with   a  considerable  quantity  of 
gravel,  in  which  case  it  is  used  for  sidewalks,  or  with 
road-metal  if  intended  for  carriage-ways.     For  the 
lirst  object  there  is  no  doubt  of  its  success,  if  it  can 
be  afforded  at  a  sufficiently  low  price ;  for  the  latter 
purpose  its  value  has  not  been  sufficiently  tested, 
13 


158  PRACTICAL   MECHANICS. 


VIL 

EAILROADS. 


172.  The  want  of  continuity  in  the  motion  of  a 
wheel  carriage,  the  lateral  friction  upon  the  road, 
and  that  arising  from  penetration  in  soft  materials, 
may  all  be  obviated  by  causing  the  wheels  to  move 
upon  parallel  bars  laid  in  the  direction  of  the  road. 
In  this  way  also  it  is  possible  to  improve  the  struc- 
ture of  the  carriage,  and  give  it  greater  nicety  of 
workmanship. 

Railroads  were  first  introduced  in  the  mines  of 
Durham  county,  England,  and  took  their  rise  grad- 
ually, from  accidental  causes.  It  had  been  usual  to 
lay  parallel  rails  in  mines,  on  which  carriages  with 
two  wheels  were  moved  by  men.  In  the  great  works 
which  these  mines  required,  the  galleries  were  en- 
larged, until  carriages  drawn  by  horses  were  substi- 
tuted. It  was  now  found  that  a  horse  could  draw 
much  more  in  a  cart  moving  upon  rails  than  he  could 
upon  a  common  road.  The  rails  were,  in  conse- 
quence, extended  from  the  mines  to  the  wharves  at 
which  the  coal  is  shipped.  The  rails  in  this  early 
instance  were  made  of  wood,  a  flanch  was  applied 
to  them  to  prevent  the  wheels  of  the  cart  from  slip. 
ping  off  the  rail.  In  order  to  lessen  the  wear  of  the 
wood,  the  rail  was  covered  with  a  plate  of  wrought 
iron.  In  this  state  railroads  remained  for  a  centu- 
ry, and  were  confined  to  the  district  in  which  th^y 
were  originally  introduced. 

As  timber  became  scarce,  and  the  price  of  cast 


PRACTICAL    MECHANICS.  159 

iron  became  low  in  England,  the  latter  material  was 
substituted  for  the  former.  The  rails  were  cast  in 
lengths  of  about  three  feet,  with  a  flanch  on  the  in- 
ner side,  and  were  supported  at  their  junctions  on 
pillars  or  blocks  of  stone.  In  this  form  the  way  was 
called  a  tram-road.  Originally  intended  for  the  use 
of  common  carriages,  it  was  soon  discovered  that 
carriages  expressly  constructed  for  the  purpose  were 
more  advantageous.  In  these,  as  there  was  no  ne- 
cessity for  turning,  the  wheels  were  all  made  of  equal 
heights  ;  as  there  was  no  difference  in  the  height  of 
the  rails  on  opposite  sides  of  the  road,  the  wheels 
were  not  made  to  dish,  the  axle-tree  was  made 
straight  from  end  to  end,  and  was  made  to  revolve 
with  the  wheel.  In  these  tram-roads,  the  quantity 
of  weight  moved  by  a  horse  was  increased  fivefold. 

The  next  improvement  consisted  in  removing  the 
flanch  from  the  rail,  and  placing  it  on  the  inner  side 
of  the  tire  of  the  wheel.  The  surface  of  the  rail 
was  now  rounded,  either  throughout  its  whole  upper 
surface  or  at  the  edges.  Wrought  iron,  which  can 
be  obtained  in  a  straight  form  for  several  yards  to- 
gether, was  substituted  for  cast  iron,  whose  length  in  a 
straight  piece  is  limited.  The  most  customary  form 
of  this  description,  the  edge  rail,  has  a  section  of  the 
figure  T,  and,  when  curves  are  frequent  in  the  road, 
the  shape  of  an  ffi  was  employed.  In  the  United 
States,  rails  of  wood,  merely  faced  with  iron,  were 
adopted  in  consequence  of  the  cheapness  of  the 
method. 

173.  After  the  experience  of  some  years,  it  seems 
to  be  universally  admitted  that  this  combination  of 
materials  forms  the  best  railroads.  The  elasticity 
of  the  wood  appears  to  act  as  a  spring,  yielding  at 
first  to  the  shock  of  the  heavy  weights  which  are 
moved  upon  it,  and  then  restoring  itself.     The  road 


160         PRACTICAL  MECHANICS. 

of  this  combined  material  is  therefore  less  injured  by 
the  traffic  upon  it,  and  the  carriages  which  travel 
upon  it  last  longer.  Of  all  the  forms  of  railroad,  that 
which  is  composed  of  plates  of  iron  laid  on  continu- 
ous lines  of  stone  is  the  worst,  in  consequence  o. 
its  possessing  no  elasticity  whatever.  Rails  ofwooc 
are,  however,  liable  to  the  important  objection  of  want 
of  durability.  It  has  been  proposed  to  obviate  this 
by  the  process  of  Kyan,  in  which  an  insoluble  com- 
pound is  formed  of  corrosive  subUmate  with  the  al- 
bumen of  the  wood.  From  experiments  which  we 
have  witnessed,  it  appears  that  this  combination  only 
takes  place  at  the  mere  surface  of  the  wood,  except 
at  the  ends,  and  here  the  penetration  does  not  exceed 
an  eighth  of  an  inch.  It  would  therefore  appear 
probable,  that  this  method,  instead  of  preserving, 
would  rather  hasten  the  decay  of  the  heart  of  the 
wood.  Under  this  impression,  we  should  consider 
the  method  already  spoken  of,  by  which  wood  may 
be  saturated  throughout  with  bituminous  matter,  is 
to  be  preferred. 

The  road,  having  been  brought  to  the  desired  grade 
by  cutting  and  filling,  must  be  allowed  to  consolidate 
itself.  The  foundation  for  receiving  the  rails  may 
either  be  composed  of  cross  sleepers  of  wood,  or  of 
blocks  of  stone.  If  the  ground  be  not  sufficiently 
firm,  it  may  be  rendered  so  by  means  of  road-metal 
well  rammed.  The  distance  between  the  sleepers, 
and  between  the  centres  of  the  stone  blocks,  is  usual- 
ly three  feet.  Wooden  rails  may  be  dropped  into 
notches  cut  in  the  sleepers  ;  and  rails,  whether  of 
wood  or  iron,  are  supported  on  the  stone  blocks,  by 
means  of  clamp-shaped  pieces  of  cast  iron  called 
chairs.  The  chairs  are  bolted  down  to  the  stone 
blocks,  and  the  rails  are  wedged  to  the  chairs.  In 
laying   the   rails,  room  must  be  left  for  their  ex- 


PRACTICAL    MECHANICS.  161 

pansion  by  heat ;  and  in  fastening  iron  on  wood  by- 
spikes,  the  countersunk  holes  through  which  the 
spikes  pass  ought  to  be  oblong,  for  the  same  rea- 
son. 

174.  Upon  a  level  railroad  of  the  best  construc- 
tion, with  carriages  of  the  most  perfect  finish,  a 
horse-power  is  able  to  draw  a  load  of  about  22J  tons. 
Under  usual  circumstances  this  load  is  about  16  tons. 
The  resistance  to  motion  on  a  level  railroad  has 
therefore  been  reduced  as  low  as  aio^h,  and  may  be 
safely  taken  at  2 oo t^*  ^^  ^^ve  seen  that  upon  the 
best  common  road  it  is  never  less  than  4Vth,  but  is, 
in  most  cases,  as  great  as  gV^^*  '^^^  advantage  of 
a  good  railroad  over  a  common  turnpike,  when 
horses  are  employed,  is  therefore  about  10  :  1.  On 
the  other  hand,  a  horse  draws  in  a  boat  on  a  canal 
thirty  tons  ;  and  in  canals  which  admit  boats  that  are 
drawn  by  more  than  one  horse,*  at  the  rate  best 
adapted  to  the  exertion  of  this  kind  of  strength,  the 
weight  drawn  increases  in  a  higher  ratio  than  the 
number  of  horses  attached  to  a  single  boat.  When, 
therefore,  horses  are  the  prime  mover  employed,  a 
canal  has  an  advantage  in  the  transportation  of  heavy 
goods  over  a  railroad,  in  the  ratio  of  at  least  2:1, 

175.  When  speed  is  the  object  in  view,  as  in  the 
transportation  of  passengers  and  of  valuable  mer- 
chandise, the  railroad  has  the  advantage  over  the  ca- 
nal, even  when  horses  are  used  as  the  moving  power. 
Although  some  recent  experiments  have  shown  that 
the  resistance  to  motion  in  fluids  does  not  increase, 
at  higher  velocities,  in  a  ratio  near  as  great  as  has 
usually  been  slated,  still  it  does  not  appear  certain 
that  a  speed  greater  than  ten  miles  an  hour  can  be 
kept  up  upon  a  canal  by  the  draught  of  horses.  On 
the  other  hand,  horses  have  drawn  cars  loaded  with 


162  PRACTICAL    MECHANICS. 

passengers,  on  the  Camden  and  Amboy  Railroad,  at 
an  average  rate  of  fifteen  miles  per  hour. 

176.  Railroads  derive  their  greatest  value  from 
the  use  of  steam  as  the  moving  power.  The  intro- 
duction of  this  agent  was  attempted  upon  them  at  an 
early  period  in  their  history,  but  the  first  experi- 
ments were  unsuccessful,  and  the  engines  used  for 
the  purpose  are,  even  at  the  present  day,  receiving 
continual  improvements.  Trevithick,  who  was  the 
first  to  apply  steam  to  locomotion,  made  use  of  the 
very  principle  which  is  now  successfully  employed, 
but  failed,  partly  in  consequence  of  the  imperfect 
state  of  railroads  at  the  time,  and  partly  in  conse- 
quence of  his  not  giving  the  principle  all  the  exten- 
sion of  which  it  is  capable.  He,  in  fact,  made  use 
of  no  more  than  one  fourth  of  the  tractive  power  of 
which  his  engine  was  capable. 

177.  Two  principal  methods  have  been  proposed 
for  the  propulsion  of  carriages  upon  railroads  by 
means  of  steam,  namely,  stationary  and  locomotive 
engines.  Stationary  engines  are  set  up  in  buildings 
on  the  sides  of  the  roads,  and  their  action  is  convey- 
ed to  the  cars  by  means  of  ropes  or  chains.  This 
method  is  attended  with  many  inconveniences,  and 
has  therefore  never  been  used  except  for  surmount- 
ing considerable  changes  of  level  within  a  short  dis- 
tance ;  and,  even  in  this  case,  the  delay  which  at- 
tends their  passage  has  led  to  the  laying  out  of  rail- 
roads in  such  manner  as  to  avoid  their  use  as  far  as 
possible. 

178.  In  locomotive  engines,  after  the  abortive  at- 
tempt of  Trevithick,  an  apparatus  resembling  in 
structure  the  human  leg  was  tried,  but  unsuccessfully. 
A  more  feasible  plan  was  that  of  adapting  a  fifth 
wheel,  cut  into  teeth,  to  the  car,  and  causing  it  to 


PRACTICAL    MECHANICS.  163 

catch  into  the  teeth  of  a  rack  laid  parallel  to  the  rails. 
This  method  was  successfully  used  near  Leeds,  in 
England,  for  several  years,  in  the  transportation  of 
coal,  and  might  still  be  employed  in  cases  where 
heavy  weights  are  to  be  moved  with  small  velocities, 
or  raised  up  steep  inclinations.  On  the  other  hand, 
the  continuity  of  motion  is  not  preserved,  and  the 
most  important  advantage  to  be  derived  from  the  use 
of  steam  is  excluded.  The  method  which  has  now 
superseded  all  others  is  as  follows  :  Two  or  more 
of  the  wheels  of  a  carriage  being  made  to  revolve  by 
an  engine  mounted  upon  it,  the  carriage  will  be  car- 
ried forward  in  consequence  of  the  friction  of  these 
wheels  upon  the  rails.  Now,  as  has  been  shown, 
the  least  friction  of  the  cars  on  a  well-constructed 
railroad  has  not  been  found  less  than  ^l^th,  and  can- 
not be  safely  estimated  under  ordinary  circumstan- 
ces at  less  than  ^^o^^*  1"^^  locomotive  engine  is 
farther  resisted  by  a  friction  growing  out  of  the 
pressure  of  the  train  upon  the  axles  of  its  wheels. 
This  friction  is  estimated  at  1  lb.  per  ton.  To  these 
resistances  are  to  be  added  that  of  the  air,  amount- 
ing, at  a  velocity  of  twenty  feet  per  second,  to  nearly 
1  lb.  on  every  square  foot  of  the  front  of  the  leading 
carriage. 

To  overcome  these,  we  have  the  friction  of  iron 
upon  iron  at  the  points  where  the  wheels  touch  the 
rail.  This  friction  is  about  ^ths  of  the  pressure. 
But,  as  dust  or  other  moveable  substances  may  inter- 
vene, as  the  rail  may  be  moistened  with  rain  or  dew, 
or  even  coated  with  ice,  the  efficient  adhesion  cannot 
be  safely  taken  at  much  more  than  |th.  Admitting 
this  fraction  for  the  measure  of  the  adhesion,  and  ^Joth 
for  the  measure  of  the  resistances,  a  locomotive  ought 
to  be  able  to  drag  after  it  any  weight  less  than  32 
times  that  which  rests  on  the  wheels  which  are  driv- 


164  PRACTICAL    MECHANICS 

en  by  the  engine  ;  and  there  are  instances  in  which 
an  engine  has  drawn  50  times  its  own  weight.  In 
practice,  however,  the  load  is  usually  limited  to  25 
times  the  pressure  which  the  locomotive  exerts  on  the 
rails,  when  moving  with  a  velocity  of  12 J  miles  per 
hour,  and  with  steam  of  the  expansive  force  of  50 
.bs.  per  inch. 

179.  It  is  easy,  by  means  of  a  crank,  to  cause  two 
of  the  wheels  of  a  locomotive  engine  to  revolve,  and 
two  others  may  be  readily  made  to  move  with  them 
by  connecting  rods.  It  is,  however,  so  difficult  to 
give  exactly  the  same  diameter  to  all  the  wheels,  that 
it  has  been  often  found,  that  of  the  two  wheels  united 
by  a  connecting  rod,  one  alone  was  efficient,  and 
hence  it  is  usual  to  restrict  the  number  of  wheels 
driven  by  the  engine  to  two.  On  this  account,  cur- 
ricle engines,  in  which  the  whole  weight  is  carried 
on  no  more  than  two  wheels,  were  at  one  time  tried, 
but  were  not  approved  of.  The  most  efficient  loco- 
motive engines  which  are  at  present  in  use,  rest  upon 
six  wheels.  Two  of  these  are  much  larger  than  the 
others,  and  are  driven  by  the  engine.  In  England, 
these  wheels  are  placed  between  the  others.  In  this 
country  the  four  small  wheels  are  combined  in  a  sin- 
gle frame  under  one  end  of  the  carriage,  and  the  other 
end  rests  on  the  two  large  wheels.  The  first  engine 
of  the  latter  description  was  planned  for  the  use  of 
the  Mohawk  and  Hudson  Railroad,  by  Mr.  J.  B. 
Jervis,  at  that  time  chief  engineer  of  that  work,  as 
long  since  as  1826. 

180.  A  locomotive  engine  is  in  all  cases  propelled 
by  steam  of  high  pressure.  The  reasons  which  for- 
bid the  use  of  the  condensing  engine  have  been  sta- 
ted in  §  119.  The  cylinder  of  the  engine  may  be  ei- 
ther horizontal  or  vertical,  and  in  many  cases  has 


PRACTICAL   MECHANICS. 
Fig.  55. 


165 


been  placed  in  an  inclined  position.  Two  cylinders 
are  generally  employed.  These  not  only  serve  to 
adjust  the  weight  more  conveniently,  but  they  may  be 
geared  to  cranks  placed  upon  the  same  axle,  at  right 
angles  to  each  other,  and  thus  one  will  be  at  the  mid- 
dle of  its  stroke,  and  therefore  acting  with  its  great- 
est intensity,  at  the  time  that  the  other  is  passing  the 
centre.  The  motion  will  be  rendered  in  this  way 
more  equable.  To  the  piston-rod  of  each  cylinder 
a  connecting  rod  is  adapted,  which  is  applied  at  its 
other  extremity  to  a  crank  on  the  axle  of  one  of  the 
pairs  of  wheels  on  which  the  engine  is  carried.  This 
nair  of  wheels  is  thus  caused  to  revolve. 

An  engine  of  the  latest  construction,  with  six 
wheels,  the  two  larger  of  which  are  driven  by  a  cyl- 
inder on  each  side  of  the  boiler,  is  represented  Fig 


65. 


14 


166  PRACTICAL    MECHANICS. 

181.  As  the  resistance  upon  a  railroad,  under  the 
most  favourable  circumstances,  is  ^ io^l^j  ^^d  sls  it 
does  not  amount  on  any  well-constructed  railroad  to 
more  than  2^o-l^>  i^  follows  that  the  greatest  slope 
which  can  be  admitted  upon  a  road,  without  an  in- 
crease in  the  moving  power,  or  a  diminution  in  the 
load,  is  1  foot  in  200,  or  26  feet  per  mile.  Rail 
roads,  therefore,  wherever  economy  in  the  cost  of 
transportation  is  the  principal  object,  are  to  be  laid 
out  in  a  series  of  levels  or  slopes  not  exceeding  ^^  q. 
When  this  mode  of  laying  out  the  road  is  adopted, 
the  several  levels  are  united  by  inclined  planes. 
When  passengers  are  conveyed  on  such  roads  with 
great  velocities,  the  power  of  the  engine,  compared 
with  its  load,  may  be  sufficient  to  enable  it  to  sur- 
mount the  planes  with  a  diminished  velocity.  This 
method  is  applicable  with  certainty  up  to  the  limit  of 
•j'o  th,  or  64  feet  per  mile,  and  some  of  our  American 
engines  have  performed  their  task  successfully  on 
slopes  of  yVh. 

At  greater  inclinations,  or  when  heavy  loads  are 
carried  at  small  velocities,  additional  power  may  be 
required  to  ascend  the  planes.  This  may  be  fur- 
nished, either  by  a  stationary,  or  by  an  additional  lo. 
comotive  engine.*  Whenever  the  inclination  of  the 
plane  exceeds  aVth,  it  would  be  better  to  resort  to 
stationary  engines  ;  but  as  their  use  is  a  great  cause 
of  delay,  engineers  usually  endeavour  to  give  a  less 
slope  to  their  inclined  planes,  and  thus  render  them 
capable  of  being  surmounted  by  a  locomotive  en- 
gine. 

Even  a  slope  of  26  feet  per  mile  cannot  be  over- 
come without  a  diminution  in  the  velocity,  and  this 
will  begin  to  be  distinctly  obvious  whenever  the  slope 
exceeds  ^i^,  or  13  feet  per  mile. 

The  locomotives  used  for  this  purpose  are  he/'V}^ 


PRACTICAL    MECHANICS.  167 

and  all  their  wheels  are  caused  to  turn  by  the  engine, 
in  order  that  all  their  several  adhesions  may  concur 
in  the  effect.  Engines  constructed  for  this  purpose 
are  called  bank  engines,  and  one  is  stationed  at  each 
inclined  plane. 

182.  When  the  greater  part  of  the  trade  on  a  rail- 
road is  in  the  descending  direction,  the  loaded  cars 
may  be  made  to  draw  up  such  as  are  empty.  In  the 
latter  case,  it  is  necessary  to  apply  means  for  pre- 
venting the  acceleration  of  the  descending  cars.  The 
best  method  yet  employed  for  this  purpose  is  that  in- 
troduced by  Mr.  J.  B.  Jervis,  on  the  railroad  of  the 
Hudson  and  Delaware  Canal  Co.  It  is  composed  of 
a  wheel  revolving  on  a  vertical  axis,  and  furnished 
with  leaves,  whose  motion  through  the  air  is  resisted. 
It  is,  in  fact,  the  method  of  a  fly  with  leaves,  referred 
to  in  §  16.  Friction  may  also  be  applied  by  a  brake 
to  the  wheels  of  each  car ;  but  this  method  requires 
several  men  for  its  application,  while  the  former  is 
self-acting. 

183.  Railroads  ought  not  only  to  be  nearly  level, 
but  also  as  straight  as  possible.  Curved  railroads 
are  objectionable  for  several  reasons : 

(1.)  The  axles  have  so  little  play,  that,  in  turning 
in  curves,  one  of  the  wheels  on  each  axle  must  drag 
or  pass  along  the  rail  by  sliding,  as  well  as  by  its 
revolution.  The  resistance  is  in  this  case  material- 
ly increased. 

(2,)  In  changing  the  direction  of  the  course  of 
cars,  there  is  a  great  risk  incurred  of  their  running 
off  the  rails. 

(3.)  A  centrifugal  force  is  produced  at  the  curves, 
by  which  a  pressure  takes  place  on  the  outer  rail, 
causing  an  increase  in  the  lateral  friction,  and  a  ten- 
dency to  spread  the  rails  or  separate  them  from  each 
other. 


168         PRACTICAL  MECHANICS. 

In  changing  the  direction  of  the  lines  of  railroads, 
the  curves  must,  in  consequence,  be  made  of  the  lar- 
gest possible  radius  ;  and  to  lessen  the  action  of  the 
two  last  of  the  above  causes,  the  outer  rail  should  be 
raised  above  the  level  of  the  other.  The  amount  of 
this  elevation  will  depend  on  the  breadth  of  the  track, 
the  radius  of  the  curve,  and  the  velocity  of  the  car.* 
If  the  velocity  be  thirty  miles  per  hour,  the  elevation 
of  the  outer  rail,  in  a  track  of  mean  width,  should 
be  nearly  thirteen  inches  when  the  radius  of  the 
curve  is  250  feet ;  with  the  same  velocity,  the  eleva- 
tion becomes  6^  inches  at  500  feet  radius ;  3^d 
inches  at  1000  feet ;  Ifds  at  2000  feet. 

It  is  also  usual  to  make  the  tire  of  the  wheels 
slightly  conical  when  curves  frequently  occur  on  a 
railroad  ;  this  gives  facility  in  changing  the  direction 
of  the  carriage,  and  moderates  the  centrifugal  force. 
This  method,  however,  is  not  without  objection  upon 
the  straight  parts  of  the  road.  A  method,  which  is 
considered  better,  consists  in  uniting  the  flanch  v/ith 
the  rest  of  the  tire  by  a  conical  surface.  This  has 
all  the  advantage  of  a  conical  wheel  at  the  curves, 
and  is  not  liable  to  objection  on  the  straight  parts  of 
the  road. 

184.  Tram-roads  being  intended  for  the  use  of 
common  carriages,  the  distance  between  the  rails 
was  made  the  same  as  that  of  an  ordinary  wheel  track, 
say  four  feet  eight  inches.  Although  no  such  rea- 
son applies  to  railroads  in  their  present  state,  it  has 
still  been  usual  to  limit  the  space  between  the  rails 
to  this  distance.  It  would,  however,  appear  that  there 
IS  no  good  reason  for  this  practice,  and  there  are  ob- 
vious advantages  to  be  derived  from  an  increase  in 
the  width  of  the  track.     The  height  of  the  wheels 

*  See  Pambour  on  Railroads. 


PRACTICAL  MECHANICS.         169 

must  bear  such  a  relation  to  this  width  as  will  pre- 
vent the  equilibrium  of  the  cars  from  becoming  un- 
stable, and  this  gives  a  limit  to  the  radius  of  the 
wheel,  considered  as  a  lever,  which  prevents  its  be- 
ing as  effectual  in  overcoming  the  friction  as  it  ought 
to  be.  An  increase  in  the  diameter  of  the  driving, 
wheels  of  locomotives  will  also  be  favourable  to  ve- 
locity with  a  given  force  of  steam.  For  these  rea- 
sons, on  a  railroad  recently  made  in  Russia,  the 
breadth  of  the  track  has  been  made  six  feet ;  and  in 
the  Western  Railroad,  in  England,  the  breadth  has 
been  made  as  great  as  eight  feet.  The  latter,  how- 
ever,  appears  to  be  excessive. 

185.  It  might  at  first  appear  that,  as  the  weight 
which  can  be  drawn  bears  an  exact  ratio  to  the 
weight  of  the  locomotive,  the  heavier  the  latter  is 
made  the  better.  The  attendance  upon  a  large  and 
small  engine  is  the  same,  and  thus  the  expense  of 
the  larger  engine  is  less  in  proportion.  Engineers 
were  for  a  time  misled  with  this  view  of  the  subject. 

It  is  to  be  considered  that  every  increase  in  the 
weight  of  the  engine  is  attended  with  increased  wear 
and  tear  in  the  roads  and  in  the  engine  itself,  and  it 
has,  in  consequence,  been  inferred  that  it  is  better  to 
diminish  the  weight  of  the  locomotive  to  the  lowest 
limit  consistent  with  strength  and  the  efficient  gener- 
ation of  the  steam  required  for  working  it. 

Two  conditions  are  to  be  observed  in  planning  an 
engine  to  draw  a  given  load  ; 

(I.)  That  there  shall  be  such  proportion  between 
the  boiler  and  the  cylinder  as  shall  furnish  steam  of 
the  necessary  pressure  at  the  required  velocity. 

(2.)  That  the  weight  of  the  engine  be  sufficient 
to  give  the  adhesion  which  is  in  dynamical  equilibrio 

Ith  this  pressure. 
If  the  power  of  the  steam  exceed  the  state  of  equi- 


170  PRACTICAL    MECHANICS. 

librium  with  the  pressure,  all  such  excess  will  be 
lost ;  and  if  the  pressure  be  in  excess,  ail  such  pres- 
sure is  a  useless  load. 

In  locomotive  engines  intended  for  rapid  motion, 
it  has  been  found  most  advantageous  to  make  use  of 
the  pressure  on  no  more  than  one  pair  of  wheels. 
On  the  other  hand,  when  the  power  of  ascending 
planes  is  required,  not  only  are  heavy  engines  best, 
but  all  the  wheels  must  be  so  connected  as  to  derive 
motion  from  the  engine. 

It  has  not  been  found  expedient  to  reduce  the 
weight  of  a  locomotive  below  eight  or  nine  tons,  in- 
cluding the  water  in  the  boiler  and  all  necessary  ac- 
cessories. The  fuel  and  water  of  supply  are  carried 
on  a  separate  car,  denominated  the  Tender. 

With  an  engine  of  the  weight  of  8  tons,  the  max- 
imum load  has  been  as  much  as  175  tons,  with  the 
velocity  of  12J  miles  per  hour. 

In  doubling  the  velocity,  the  load  is  diminished  to 
one  eighth,  while  the  expenditure  of  fuel  in  a  given 
distance  is  only  lessened  one  half. 

As  an  instance  of  good  performance  of  a  locomo- 
tive engine,  we  may  cite  that  of  one  constructed  by 
Messrs.  H.  R.  Dunham  &  Co.,  of  New-York,  on 
the  Harlem  Railroad.  The  whole  weight  of  this 
engine  was  20,400  lbs.,  or,  as  nearly  as  possible,  9 
tons,  when  the  boiler  was  filled  with  water  and  the 
engine  in  working  order.  Of  this  weight  10,680 
lbs.  bore  on  the  driving-wheels.  The  load  drawn 
was  105  tons,  loaded  upon  35  cars,  whose  weight  is 
not  given.  The  ascent  overcome  on  parts  of  the 
voad  was  between  25  and  30  feet  per  mile. 


PRACTICAL   MECHANICS.  '  171 


VIII. 

CANALS   AND   DOCKS. 

186.  Canals  are  artificial  cliannels  for  the  con- 
veyance  of  water,  and  their  most  important  use  is  for 
purposes  of  navigation.  They  may  be  applied  to 
this  object  in  three  different  cases,  namely:  1.  To 
form  a  communication  between  two  navigations  upon 
the  same  level,  or  one  of  which  is  higher  than  the 
other,  drawing  their  supply  of  water  from  one  or  both 
of  these  navigable  waters  ;  2.  As  a  substitute  for 
a  stream  which  is  not  itself  navigable,  in  consequence 
of  obstructions  or  of  too  great  rapidity;  or,  3.  To 
form  a  communication  between  navigations,  both  of 
which  are  lower  than  the  ground  over  which  the  ca- 
nal must  necessarily  pass.  The  latter  case  is  the 
most  important  in  practice,  and  a  canal  of  this  de- 
scription is  said  to  have  a  summit  level,  or  to  be  d 
point  de  pariage.  The  possibility  of  passing  canals 
over  ground  more  elevated  than  the  navigations  they 
were  intended  to  unite  was  first  pointed  out  by  Ri- 
quet,  and  put  into  practice  in  the  Canal  of  Languedoc. 

187.  When  a  canal  is  of  this  description,  it  is  ne- 
cessary that  the  water  for  its  supply  should  be  col- 
lected in  reservoirs,  or  carried  by  feeders  to  the  point 
whence  it  is  to  flow  in  opposite  directions.  These 
feeders  must  be  cut  along  the  slopes  of  the  higher 
o-rounds,  in  such  manner  as  to  intercept  all  the 
streams  and  surface-water  that  flow  over  them.* 

*  The  mode  of  calculating  the  slopes  and  areas  necessary  for 
conveying  the  required  quantity  of  water  may  be  seen  in  the  au- 
,  thor's  Treatise  on  Mechanics,  book  vi.,  chap.  vi. 


172  PRACTICAL    MECHANICS. 

188.  The  quantity  of  water  which  may  be  inter- 
cepted by  a  feeder  is  ascertained  by  gauging  the 
streams,  or  estimating  the  quantity  of  rain  which 
falls  upon  the  surface  whose  slope  is  directed  to  the 
same  feeder.* 

189.  The  dimensions  of  a  canal  maybe  determin- 
ed from  certain  considerations  of  circumstances  un- 
der which  they  are  placed.  When  they  are  intended 
to  join  two  artificial  navigations,  their  dimensions 
must  not  exceed  the  smaller  of  the  two,  otherwise 
transhipment  may  be  necessary  at  the  point  of  junc- 
tion. When  they  unite  two  natural  navigations,  they 
should  be  constructed  to  accommodate  the  smallest 
class  of  vessels  which  can  safely  navigate  them,  un- 
less the  amount  of  trade  be  insufficient  to  warrant  the 
expense.  Thus  the  Raritan  and  Delaware  Canal 
has,  with  great  propriety,  been  adapted  to  the  pas- 
sage of  river-craft  of  80  tons.  But,  when  no  such 
considerations  need  be  taken  into  account,  the  best 
size  for  canals  is  that  suited  for  vessels  which  may 
be  drawn  by  a  single  horse.  These  carry  35  tons, 
and  may  be  conveniently  made  of  the  following  di- 
mensions :  length  60  feet,  beam  8  feet,  draught  of 
water  3  feet. 

190.  The  area  of  a  canal  must  be  such  as  will 
permit  two  boats  to  pass  each  other,  although  a  third 
may  be  lying  near  the  side.  The  depth  must  be  one 
foot  greater  than  the  draught  of  the  vessels,  in  order 
that  they  may  run  no  risk  of  touching  the  ground. 
The  bottom  is  level,  and  the  sides  have  such  a  slope 
as  the  earth  of  which  they  are  composed  naturally 
assumes.  The  breadth  at  bottom  is  therefore  usual- 
ly twice  as  great  as  the  beam  of  the  vessels.     The 

*  The  method  of  gauging  streams  will  be  found  in  the  author's 
Treatise  on  Mechanics,  book  vi,,  chap  vi. 


PRACTICAL  MECHANICS.         173 

janal  is  included  between  two  banks,  which  may,  ac- 
cording to  circumstances,  be  cut  in  the  ground,  or 
brmed  by  embankment.  One  of  these  is  used  as  a 
,owing-path  for  the  horses  which  draw  the  boat :  its 
)readth  at  top  must  not  be  less  than  six  feet,  and  it 
)ught  to  be  covered  with  good  materials  for  roads. 
The  other  bank  need  have  no  greater  thickness  than 
s  necessary  to  resist  the  action  of  the  water  in  the 
;anal.  When  the  natural  earth  is  retentive  of  water, 
he  banks  are  formed  of  it  alone.  When  it  is  not, 
he  canal  is  either  lined  with  an  earth  retentive  of 
vater,  or  a  vertical  layer  of  such  earth  is  worked  up 
n  the  middle  of  the  bank.  This  mode  of  lining  is 
lalled  puddling.  The  best  puddling  material  is  a 
jravelly  loam.  Clay  will  not  answer  the  purpose,  as 
t  will  not  resist  the  action  of  moving  water.  The 
)anks  of  the  Erie  Canal  have  in  many  places  been 
ined  with  a  pavement  of  rolled  pebbles. 

A  level  surface  or  berm  ought  to  be  left  between 
he  surface  of  the  water  in  the  canal  and  the  two 
)anks,  in  order  to  prevent  the  earth  from  falling  from 
hem  into  the  canal.  A  ditch  should  be  cut  on  each 
dde  of  the  canal  in  level  ground,  and  on  its  upper 
dde  in  side-lying  grounds,  in  order  to  prevent  sur- 
ace-water  from  running  into  the  canal.  The  sur- 
kee-water  which  accumulates  in  the  ditches  must  be 
)assed  under  the  canal  from  time  to  time,  in  pas- 
jages  acting  like  inverted  siphons.  These  are  called 
julverts. 

191.  A  canal,  whatever  be  the  height  of  its  sum- 
nit  level,  is  laid  out  in  a  series  of  levels.  These 
evels  may  be  made  to  communicate  with  each  other 
3y  means  of  locks  or  of  inclined  planes.  The  for- 
mer are  used  when  the  differences  of  level  are  not 
^reat,  and  the  practicability  of  a  canal  is  usually 
udged  of  in  reference  to  this  method  ;  but  the  inch- 


174  PRACTICAL    MECHANICS. 

ned  plane  has  at  last  been  successfully  used  in  the 
Morris  Canal,  and  the  possibility  of  passing  canals 
through  naountainous  regions  is  thus  established  b} 
actual  experiment. 

A  lock  is  a  chamber  usually  formed  of  two  walls 
of  masonry,  and  closed  at  each  end  by  gates.  The 
top  of  both  gates  rises  as  high  as  the  surface  of  the! 
water  in  the  higher  level  of  the  canal.  The  lower' 
gate  has  its  sill  on  a  level  with  the  bottom  or  lower 
reach  of  the  canal,  and  the  sill  of  the  upper  gate  is 
usually  established  upon  a  wall  rising  like  a  step  to 
the  bottom  of  the  upper  reach.  This  breast-wall  is 
the  weakest  part  of  the  lock,  and  in  modern  Ameri- 
can locks  it  has  been  suppressed.  The  change  of 
level  in  the  bottom  of  the  canal  is  then  made  gradu- 
ally above  the  site  of  the  upper  gate.  For  this  im- 
provement the  world  is  indebted  to  the  late  Canvass 
White,  and  it  is  the  only  important  step  in  lock  nav- 
igation made  since  the  construction  of  the  CanaL  of 
Languedoc. 

The  gates  of  canals  are  usually  made  in  two 
leaves,  meeting  at  an  angle  in  the  middle  of  the  lock. 
This  angle  is  pointed  towards  the  upper  level,  and 
its  most  advantageous  dimension  is  120°. 

Paddle.gates  are  formed  in  these  gates  for  the 
passage  of  water  from  the  upper  level  into  the  lock, 
and  from  the  lock  into  the  lower  level  of  the  canal, 
vyulverts,  furnished  with  gates  for  the  same  purpose, 
are  also  sometimes  made  in  the  walls  of  the  lock. 
These  were  absolutely  necessary  for  the  upper  gate 
before  the  improvement  of  White,  as  the  water,  run- 
ning through  an  opening  in  the  upper  gate,  might 
have  spouted  into  a  boat  occupying  the  lock.  Such 
culverts  weaken  a  lock,  and  therefore  should,  if  pes- 
sible,  be  dispensed  with. 

When  the  lower  gate  is  shut,  and  water  passed 


P,RACTICAL  MECHANICS.         175 

through  the  paddle-gate  from  the  upper  level,  the 
lock  may  be  filled  with  water.  The  pressure  on  the 
opposite  sides  of  the  upper  gate  will  then  become 
equal,  and  it  may  be  opened,  while  the  lower  gate  is 
kept  tightly  shut  by  the  pressure  of  the  water  m  the 
lock.  By  shutting  the  upper  gate,  and  allowmg  wa- 
ter  to  escape  through  the  paddle-gate  into  the  lower 
level,  the  lock  may  be  emptied,  and  the  lower  gate 
beino-  under  equal  pressures,  may  be  opened.  Ves- 
sels  °may  therefore  be  drawn  in  the  two  cases  from 
the  two  levels,  and  alternately  raised  and  lowered, 
within  the  lock,  from  the  one  to  the  other. 

The  alternate  filling  and  emptying  of  a  lock  takes 
about  ten  minutes,  and  thus,  in  a  canal  fitted  for  30 
ton  boats,  360,000  tons  may  be  passed  through  the 
locks  in  each  direction  in  the  course  of  a  year. 
This  exceeds  the  traffic  on  the  most  frequented  ca- 
nal, and  therefore,  even  if  of  this  small  size,  it  will  be 
sufficient  for  any  practical  purpose.  On  the  other 
hand,  a  greater  weight  will  be  drawn  upon  large  ca- 
nals by  a  given  number  of  horses  than  upon  small 
canals,  for  the  resistance  to  boats  of  similar  figures 
increases  only  with  the  squares  of  their  lineal  dimen- 
sions, while  their  burden  increases  with  the  cubes.  ^ 

The  course  of  trade  in  the  Northern  States  di- 
minishes the  capacity  of  the  canals  for  transportation 
very  materially.  In  the  autumn,  towards  the  close 
of  the  navigation,  the  agricultural  products  of  the  in- 
terior are  accumulated  in  great  quantities,  and  crowd 
the  canals,  while,  at  the  same  time,  foreign  manufac 
tures  and  objects  of  consumption  are  hurried  from 
the  seaports,  in  order  to  supply  the  winter's  demand. 
Early  in  the  spring,  the  merchandise  which  has 
accumulated  during  the  winter  also  seeks  its  mar- 
ket,  at  the  earliest  possible  period.  For  this  reason, 
in  the  autumn  just  before  the  navigation  closes,  and 


176  PRACTICAL    MECHANICS. 

in  the  spring  immediately  after  it  opens,  our  canals 
are  insufficient  for  the  transportation  of  the  vessels. 
At  other  seasons  the  locks  are  almost  idle.  It  thus 
happens,  that  although  as  many  tons  have  passed 
locks  on  a  30  ton  canal  in  England  as  pass  those  on 
the  Erie  Canal,  where  the  vessels  have  a  burden  of 
60  tons,  there  is  much  more  complaint  of  delay  on 
the  latter  than  on  the  former.  This  complaint  has 
led  to  a  resolution,  on  the  part  of  the  State  of  New- 
York,  to  enlarge  that  canal,  and  place  two  locks  side 
by  side,  at  each  change  of  level.  It  is  foreign  to  our 
parpose  to  inquire  how  far  such  an  additional  ex- 
pense is  warranted  by  the  circumstances  of  the  case, 
or  likely  to  return  an  adequate  income.  It  is,  how- 
ever, certain  that  the  cost  of  transportation  will  be 
lowered,  and  room  will  therefore  be  left  for  an  in- 
crease in  the  tolls. 

192.  The  difference  of  level  which  may  be  over- 
come by  a  single  lock  will  depend  upon  the  cost  of 
construction,  and  the  quantity  of  water  required  to 
fill  it.  The  latter  increases  with  every  increase  of 
the  height  of  the  lock ;  the  former  is  a  minimum  be- 
tween the  heights  of  eight  and  ten  feet.  All  the 
locks  on  either  side  of  the  summit  level  of  a  canal  are 
usually  made  of  the  same  height,  in  order  that  the 
water  discharged  from  one  shall  exactly  fill  that  be- 
low it,  and  thus  there  may  be  no  waste,  or  no  need 
of  an  additional  supply  of  water.  A  better  rule  is  to 
make  the.  locks  diminish  in  height  from  the  place 
where  a  feeder  enters,  in  order  that  the  other  causes 
of  waste  of  water  may  be  compensated. 

When  the  difference  of  level  between  two  parts  of 
a  canal  is  greater  than  can  be  overcome  by  a  single 
lock,  the  locks  must  not  be  placed  in  juxtaposition, 
otherwise  a  single  boat  will  occupy  the  system  for  the 
length  of  time  necessary  to  pass  all  the  several  locks, 


PRACTICAL    MECHANICS.  177 

and  the  expenditure  of  water  will  be  proportioned  to 
their  number.  Nor  must  the  intervening  space  be 
limited  to  that  necessary  for  two  boats  to  pass,  oth- 
erwise the  quantity  of  water  drawn  to  fill  a  lock 
might  leave  the  boats  aground.  When  circumstan- 
ces compel  the  engineer  to  place  locks  in  juxtapo- 
sition, the  system  ought  to  be  double,  so  that  one  set 
may  be  occupied  by  the  ascending,  the  other  by  the 
descending  vessels. 

193.  In  the  inclined  plane  proposed  by  Fulton,  the 
boats,  being  placed  on  carriages  while  in  the  water, 
were  drawn  over  a  ridge  having  a  slope  in  both  di- 
rections, by  a  force  derived  from  a  vessel  of  water 
descending  in  a  vertical  well.  A  similar  double 
plane  was  used  by  Kitchell  on  the  Morris  Canal,  but 
the  power  was  derived  from  a  water-wheel.  In  the 
inclined  plane  of  the  Duke  of  Bridgewater's  mines, 
the  boats  passed  from  the  upper  level  into  locks,  on 
the  emptying  of  which  they  rested  on  the  carriages ; 
the  trade  being  a  descending  one,  the  loaded  boats 
draw  up  the  empty  ones.  In  the  inclined  planes  now 
in  use  on  the  Morris  Canal,  the  method  of  locks  at 
the  head  of  the  plane  is  imitated  ;  but  as  the  trade  is 
alternating,  the  power  is  derived  from  a  water-wheel. 

Water-wheels  are  objectionable  as  a  power  for  the 
inclined  planes  of  a  canal,  because  they  require  a 
continual  supply  of  water,  which,  at  heights  of  more 
than  40  feet,  may  exceed  that  necessary  to  fill  a  lock. 
A  water  counterpoise  moving  on  a  parallel  inclined 
plane,  where  the  quantity  of  water  necessary  to  set 
the  system  in  motion  would  continue  the  motion 
through  any  change  of  level  whatever,  is  therefore 
preferable.  This  is  the  method  which  was  proposed 
by  the  author  in  the  original  project  of  the  Morris 
Canal. 

194.  The  supply  of  water  for  a  canal  depends 


178         PRACTICAL  MECHANICS, 

upon  the  quantity  required  for  lockage,  the  evapora- 
tion from  the  surface,  the  leakage  through  the  banks 
and  through  the  joints  of  the  gates.  In  respect  to 
lockage,  a  lock  full  of  water  is  allowed  for  every  boat 
which  will  probably  pass,  although  one  is  sufficient 
for  letting  one  boat  up  and  another  down.  The  ex- 
cess of  evaporation  over  rain  is  usually  taken  at  a 
depth  of  three  feet  on  the  surface  of  the  canal  in  a 
year.  The  leakage  through  the  banks  is  estimated 
at  double  this  amount.  It  is  sufficient  to  allow  for 
the  leakage  of  one  gate  on  each  side  of  the  summit 
level,  as  the  water  which  thus  escapes  from  one  gate 
is  caught,  and  supplies  the  leakage  of  that  beneath  it. 
In  practice  in  the  United  States,  the  mode  of  esti- 
mating the  necessary  supply  of  water  which  has  just 
been  stated,  is  said  to  be  far  from  sufficient.  The 
engineers  who  are  employed  on  the  Erie  Canal  have 
stated  officially,  that  the  demand  of  water  for  its  ser- 
vice amounts  to  100  cubic  feet  per  mile  every  min- 
ute. The  same  estimate  has  been  reached  in  the 
canals  of  the  State  of  Pennsylvania.  In  conformity 
with  the  former  estimate,  it  has  been  inferred  that 
the  Erie  Canal,  when  enlarged,  will  require  a  supply 
every  minute  of  200  cubic  feet  per  mile.  In  order 
to  convey  forward  such  large  bodies  of  water  from 
distant  sources,  it  becomes  necessary  to  give  a  slope 
to  the  bed  of  the  canal  between  the  two  locks  which 
close  each  of  its  levels  or  ponds  ;  and  the  larger  the 
pond,  the  greater  the  slope  which  must  be  given  to 
its  bed.  A  farther  increase  in  the  flow  will  be  gained 
by  making  the  canal  diminish,  both  in  breadth  at  the 
surface  and  depth,  as  the  distance  from  the  source  of 
supply  diminishes.  For  want  of  such^precautions  in 
the  outset,  much  difficulty  has  been  found  in  some 
places  in  furnishing  the  necessary  supply  of  water. 
This  is  particularly  the  case  in  the  level  which  ex- 


PRACTICAL    MECHANICS.  179 

tends  for  about  60  miles  from  Lockport  to  Rochester, 
along  which  it  was  originally  intended  to  convey 
water  from  Lake  Erie  to  supply  the  canal  for  a  dis- 
tance of  30  miles  farther  to  the  eastward. 

195.  The  other  structures  which  are  necessary 
on  canals,  are  :  waste  gates,  by  which  any  excess  of 
water  may  be  discharged  ;  waste  weirs,  by  which  it 
may  be  prevented  from  rising  above  a  proper  level ; 
these  are  prismatic  mounds  of  masonry,  whose  edge 
is  on  the  level  at  which  the  water  ought  to  be  main- 
tained ;  culverts,  by  which  streams,  whose  level  is 
nearly  the  same  with  that  of  the  canal,  may  be  pass- 
ed beneath  it ;  and  aqueducts,  having  the  form  of 
bridges,  by  which  the  canal  may  be  carried  over 
deep  valleys  and  wide  water-courses. 

Aqueducts  may  be  trunks  of  wood  resting  on  piers 
of  masonry.  These  have  the  advantage,  in  this  coun- 
try, of  saving  in  the  original  cost,  but  are  objected  to 
for  want  of  durabiUty.  It  would,  however,  appear, 
from  the  experience  of  the  Erie  Canal,  that  they  have 
lasted  as  long  as  some  of  those  built  of  stone. 

The  best  aqueducts  are  formed  of  plates  of  cast 
iron,  united  by  bolts  passed  through  flanches.  These 
may  be  supported  on  pillars  of  stone,  when  they  can 
be  placed  near  enough  to  dispense  with  the  use  of 
the  arch.  In  other  cases  they  are  supported  on 
arches  of  cast  iron.  The  finest  aqueduct  of  this  de- 
scription is  in  the  Valley  of  Llangollen,  in  Wales,  on 
the  Ellesmere  Canal. 

196.  Wet  Docks  are  basins  constructed  in  places 
where  there  is  a  considerable  fall  of  the  tide,  in  order 
to  keep  vessels  afloat  when  the  tide  ebbs.  They  are 
connected  with  the  tide-way  by  gates  resembling 
those  of  a  canal  lock,  and  these  gates  are  some- 
times two  in  number,  having  a  space  between  them 


180  PRACTICAL   MECHANICS. 

sufficient  to  contain  a  vessel.  This  space  may 
therefore  be  made  to  answer  the  purpose  of  a  lock. 
The  best  specimens  of  these  basins  are  those  in  the 
vicinity  of  London,  a  description  of  which  may  be 
found  in  "  The  Public  Works  of  Great  Britain,"  and 
those  of  Liverpool. 

197.  A  Dry  Dock  is  a  basin  into  which  a  vessel 
may  be  floated,  and  shut  up  by  gates.  The  water  is 
then  discharged  either  by  the  fall  of  the  tide,  or 
pumped  out  when  that  is  not  sufficient.  A  vessel 
may  thus  be  laid  dry  for  the  purpose  of  repair.  The 
gates  are  usually  similar  to  those  of  a  canal  lock  or 
wet  dock,  but  open  in  the  opposite  direction,  or  out- 
ward. Besides  this  kind  of  gate,  a  floating  gate  is 
often  used  in  dry  docks.  This  is  a  vessel  of  such 
length  and  depth  as  to  occupy  the  whole  opening  of 
the  dock.  The  walls  have  two  deep  grooves  cut 
opposite  to  each  other  in  the  masonry,  and  united  by 
a  horizontal  groove  at  bottom.  The  walls  are  in- 
clined towards  each  other,  so  that  the  vessel  may  lie 
lengthwise  between  them  when  afloat,  but  will  enter 
the  grooves  while  in  the  act  of  sinking,  and  fill  them 
when  it  reaches  the  bottom.  The  floating  gate,  be- 
ing introduced  in  this  place,  is  caused  to  sink  by  ad- 
mitting water  through  an  opening  in  its  bottom. 
When  the  gate  is  to  be  removed,  this  opening  is 
closed,  and  the  water  pumped  out. 

Dry  docks  are  of  absolute  necessity  for  the  repair 
of  large  vessels,  and  a  navy  cannot  be  maintained 
in  an  efficient  state  without  them. 

For  smaller  vessels,  the  Marine  Railway  of  Mor- 
ton, the  Screw  Dock,  the  Hydraulic  Dock,  and  a 
Floating  Dock  recently  introduced  in  New. York, 
may  suffice,  and  are  less  costly  than  the  Dry  Dock. 

In  Great  Britain,  where  the  tide  falls  enough  to 
permit  the  docks  to  be  emptied  by  its  ebb  alone,  and 


PRACTICAL  MECHANICS.         181 

where  the  space  on  which  the  dock  is  tc  be  con- 
structed  is  dry  at  low  water,  no  great  difficulty  ex- 
ists in  the  construction  of  a  dry  dock.  In  most 
parts  of  the  United  States,  this  facility  is  not  to  be 
found.  They  have  therefore  been  built  at  Norfolk 
and  Charlestown,  Mass.,  by  means  of  a  coffer-dam, 
enclosing  the  space  they  were  intended  to  occupy. 
This  method  appears  to  be  much  less  efficient,  and 
far  more  costly  than  that  used  under  similar  circum- 
stances at  Toulon,  where  the  dock  was  built  in  a 
large  floating  vessel  or  caisson,  and  was  sunk  by  its 
own  weight  upon  a  foundation  of  piles, 

198.  Canals  are  also  used  for  the  purpose  of  con- 
veying water  for  the  supply  of  cities,  in  which  case 
they  are  called  aqueducts.  It  generally  happens 
that,  where  ground  is  covered  with  buildings  and 
the  streets  paved,  the  springs  subside,  and  finally 
disappear.  Even  when  the  springs  derive  their  wa- 
ter from  distant  sources,  it  is  apt  to  be  contam- 
inated by  the  filth  which  the  surface-water  carries 
through  such  parts  of  the  ground  as  can  be  pene- 
trated by  it.  In  other  cases,  the  usual  supply  being 
cut  off  from  above,  water  charged  with  foreign  mat- 
ter may  pass  in,  even  if  it  do  not  rise  to  as  great  a 
height.  These  facts  have  all  been  illustrated  in  the 
city  of  New-York.  The  level  of  the  springs  has  fall- 
en, and  in  some  cases  wells,  once  abundantly  sup- 
plied, have  been  dried  up.  The  water  is  so  highly 
charged  with  organic  matter  in  the  wells  of  the  old- 
er parts  of  the  city,  as  to  become  putrid  after  a  few 
hours ;  and  springs  which  formerly  yielded  a  pure 
and  soft  water  have  become  brackish. 

199.  The  best  source  for  the  supply  of  a  city  is  a 
stream  which  has  run  for  some  distance  in  a  steady 
and  gentle  current.     Under  such  circumstances,  it 

15 


182         PRACTICAL  MECHANICS. 

is  found  that  all  saline  matter  is  precipitated,  and  or- 
ganic matter  ceases  to  be  soluble.  No  other  impu- 
rity remains  but  what  is  visible  in  the  form  of  sedi- 
ment, and  this  is  readily  removed  by  allowing  the 
water  to  remain  at  rest  for  a  time,  or  by  actual  fil- 
tration. Such  is  the  effect  of  exposure  to  the  sun 
and  air  upon  water,  however  charged  with  foreign 
matter,  that  the  superior  excellence  of  the  waters  of 
such  rivers  as  the  Nile  and  the  Mississippi  is  ac- 
knowledged by  all  who  have  tasted  them.  On  the 
other  hand,  when  water  is  actually  stagnant,  it  will 
likewise  deposite  its  earthy  salts ;  but  the  smallest 
quantity  of  organic  matter  will,  in  a  hot  climate,  ren- 
der it  unwholesome  not  only  to  those  who  drink  it,  but 
to  those  who  reside  in  the  neighbourhood. 

200.  In  conformity  with  these  facts,  the  best  mode 
of  conveying  water  for  the  supply  of  a  city  would  be 
in  a  channel  open  to  the  air.  It  ought,  moreover,  to 
be  of  such  a  depth  that  the  bed  may  be  below  the 
reach  of  frost,  and  with  such  velocity  as  will  prevent 
more  than  a  thin  crust  of  ice  to  form  at  its  surface. 

Channels  of  masonry,  however  well  built,  are  not 
only  expensive,  but  are  objectionable  in  consequence 
of  the  water  being  capable  of  dissolving  a  portion  of 
the  lime  which  is  contained  in  the  mortar,  unless  the 
joints  be  both  close  and  few  in  number.  The  best 
of  all  beds  for  the  purpose  is  one  of  retentive  earth, 
such  as  would  be  suited  for  puddling  the  banks  of  a 
navigable  canal,  but  containing  a  larger  proportion 
of  gravel,  in  order  to  resist  the  greater  velocity  of 
the  water.  One  of  the  best  instances  of  this  de- 
scription is  the  New  River,  by  which  a  great  part 
of  the  city  of  London  is  suppHed.  This  brings  the 
water  from  a  distance  of  39  miles,  and,  by  the  joint 
advantages  of  favourable  ground  and  good  engineer- 
ing, is  one  uninterrupted  line  of  canal  upon  a  con- 
stant slope. 


PRACTICAL    MECHANICS.  183 

201.  After  water  has  been  conveyed  for  some  dis- 
tance in  an  open  channel,  it  is  necessary  that  it 
should  be  permitted  to  rest  for  a  time,  in  order  to  re- 
gain the  air  which  separates  from  it  while  in  motion, 
and  deposite  the  sediment.  Reservoirs  are  also  ne- 
cessary to  equalize  the  supply,  husbanding  the  wa- 
ter when  it  is  not  demanded,  in  order  to  yield  it  when 
wanted.  One  of  the  best  reservoirs  is  that  of  Tou- 
louse, in  France.  It  is  a  large  basin,  excavated  to  a 
considerable  depth.  In  the  bottom  is  placed  a  layer 
of  large  rolled  stones,  at  as  great  a  distance  from 
each  other  as  will  permit  of  their  supporting  a  sec- 
ond layer  of  less  size.  Successive  layers,  thus  de- 
creasing in  size,  are  placed  in  the  reservoir,  until  the 
upper  beds  take  the  form  of  coarse  gravel,  and  these 
are  covered  with  sand.  The  interstices  in  this  ma- 
terial are  sufficienfc  to  hold  the  required  supply,  and 
it  is  under  the  circumstances  of  water  in  a  well,  hav- 
ing a  temperature  nearly  constant  throughout  the 
year. 

This  reservoir  is  not  supplied  by  a  canal,  but  is 
excavated  in  a  gravelly  soil,  so  near  to  the  bed  of  the 
river  Garonne  that  its  waters  filter  through  the  nar- 
row dike  of  loose  soil  which  separates  them,  and 
thus  reach  the  reservoir  clear  and  limpid. 

A  similar  plan  is  adopted  at  Glasgow,  in  Scotland. 
A  deep  trench  was  cut  in  a  gravelly  point  almost 
surrounded  by  the  Clyde.  In  this  a  tunnel  of  an 
elliptic  shape  was  laid  of  brick,  backed  and  jointed 
only  with  sand.  The  trench  was  then  filled  up. 
The  water  of  the  river  filters  through  the  gravel  and 
fills  the  tunnel,  whence  it  is  drawn  for  use. 

202.  When  a  body  of  good  water  exists  in  the 
neighbourhood  of  a  city  at  a  low  level,  or  when  it  is 
brought  from  a  distance  at  an  elevation  too  small  to 
permit  it  to  be  used,  it  becomes  necessary  to  raise  it 


184  PRACTICAL  MECHANICS. 

by  artificial  means.  One  of  the  earliest  instances  of 
this  sort  was  in  the  supply  of  a  part  of  London.  The 
water  of  the  Thames  is  fresh  at  London  Bridge,  and 
in  the  old  structure  of  that  name,  the  current  through 
the  arches  is  sufficiently  strong  to  work  an  under- 
shot water-wheel,  both  during  the  rise  and  fall  of  the 
tide.  On  the  axle  of  this  wheel  were  placed  three 
cranks,  each  of  which  worked  the  rod  of  a  pump, 
by  which  the  water  was  forced  to  the  top  of  a  lofty 
tower. 

At  Philadelphia,  the  water  of  the  Schuylkill  is 
formed  into  a  pond  by  a  dam.  This  furnishes  a  pow- 
er to  drive  breast- wheels,  by  which  a  set  of  horizon- 
tal forcing-pumps  is  worked. 

203.  As  water-power  can  only  be  obtained  in  par- 
ticular situations,  the  steam-engine  furnishes  a  meth- 
od more  universally  applicable  for  raising  water. 
This  is  made  use  of  by  the  Manhattan  Co.  of  New- 
York,  and  was  formerly  employed  in  Philadelphia. 
In  London,  the  pulling  down  of  London  Bridge  has 
destroyed  the  water-power  of  which  we  have  spoken, 
and  it  is  replaced  by  a  steam-engine.  Other  engines 
are  erected  at  different  points  on  that  river,  by  which, 
in  addition  to  the  New  River,  the  prodigal  supply 
with  which  that  city  is  furnished  is  obtained, 

204.  When  a  channel  by  which  water  is  convey- 
ed to  a  city  reaches  a  deep  valley  or  stream  which 
crosses  the  direction  of  its  course,  two  methods  sug- 
gest themselves  for  crossing  it.  The  first  and  most 
ancient  is  by  an  aqueduct  bridge  of  masonry  ;  the 
second  by  means  of  pipes  forming  an  inverted  siphon, 
in  which  water  will  rise  again  nearly  to  the  height  at 
which  it  enters.  The  first  method  was  practised  by 
the  Romans,  and  did  not  go  out  of  use  until  after  the 
age  of  Louis  XIV.     The  last  instance  of  a  stone 


PRACTICAL  MECHANICS.  185 

aqueduct  is  even  later,  for  one  was  constructed  near 
Lisbon  towards  tlie  close  of  the  last  century,  under 
the  orders  of  Pombal.  . 

The  second  method  was  not  practicable  until  the 
art  of  casting  iron  had  attained  a  certain  degreee  ot 
perfection,  and  this  art  had  not  yet  penetrated  into 

"ourcontemporaries,  who  are  aware  of  the  advan- 
ta-es  of  conveying  water  across  valleys  by  means  ot 
pipes,  have  supposed  that  the  profuse  use  of  aque. 
duct  bridges  of  masonry  by  the  Romans  grew  out 
of  their  ignorance  of  the  property  oi  water,  by  which 
it  tends  to  rise  to  the  level  of  its  source  or  head. 
The  reason  for  the  use  of  such  aqueducts  was  ditiei- 
ent.    The  Romans  were  unacquainted  with  cast  iron, 
and  were  therefore  compelled  to  make  such  water- 
pipes  as  they  could  not  avoid  the  use  of,  ot  lead. 
This  metal  is  so  rare  and  costly,  that  the  stone  aque- 
ducts  had  the  advantage  of  economy.     At  present 
this  reason  does  not  apply,  and  it  is  vastly  cheaper 
to  convey  water  over  a  valley  in  pipes  than  m  an 
aqueduct  of  masonry.     Even  where  it  is  necessary 
to  construct  arches,  a  saving  of  expense  may  be  ob- 
tained by  making  the  surface  of  the  bridge  incline 
downward  each  way  to  the  middle  of  the  valley,  and 
laying  the  pipes  upon  it.     Such  is  the  beautiful  aque- 
duct  of  Genoa,  and  such  was  the  plan  of  the  method 
of  crossing  the  Harlem  River  proposed  by  the  en- 
cineers  of  the  Croton  Aqueduct.     It  is  to  be  regret, 
ted  that  this  plan,  which  would  have  been  so  highly 
creditable  to  the  intelligence  of  our  people  and  the 
science  of  our  engineers,  should  have  been  abandon- 
ed,  under  the  compulsion  of  an  act  of  the  Legislature, 
for  one  resembling  the  exploded  stiuctures  of  the  en- 
sineers  of  ancient  Rome  and  of  the  dark  ages. 
Besides  the  objections  in  point  ok  cost,  loity  aque- 


186  PRACTICAL    MECHANICS. 

ducts  of  masonry  are  liable  to  the  farther  defect  of 
exposing  the  water  which  flows  in  them  to  freeze. 
This  has  led  to  the  abandonment  of  a  masonry  aque- 
duct raised  upon  arches,  in  a  climate  even  less  se- 
vere than  our  own.  It  is  many  centuries  since  the 
continual  interruption  caused  by  the  frost  has  led  to 
the  substitution  of  leaden  pipes  for  the  aqueduct  erect 
ed  by  the  Roman  emperors  at  Constantinople. 

205.  Water  is  wanted  in  cities  for  two  purposes  . 
the  supply  of  the  inhabitants  for  cooking,  washing, 
and  drinking,  and  the  cleansing  of  the  streets  and 
sewers.  The  former  ought  to  be  raised  to  the  high- 
est stories  of  houses,  while  the  latter  need  be  deliv- 
ered at  no  higher  level  than  the  surface  of  the  streets. 
But  a  small  quantity  of  water  is  wanted  for  the  for- 
mer purpose  compared  with  that  for  the  latter.  In 
London,  the  delivery  at  two  different  levels  is  man- 
aged in  an  easy  and  economic  way.  For  the  great- 
er part  of  the  24  hours,  the  water  is  distributed  from 
a  reservoir  whose  level  is  little  higher  than  the  most 
elevated  point  of  the  streets,  and  the  engine  raises  a 
large  body  to  that  height.  But,  for  a  few  hours  in 
each  day,  the  engine  pumps  a  less  quantity  of  water 
into  a  small  reservoir  situated  as  high  as  the  top  of 
the  loftiest  houses.  By  a  simple  stopcock,  the  pipes 
by  which  the  water  is  conveyed  may  be  put  in  com . 
munication  with  either  of  the  reservoirs.  While  in 
communication  with  the  larger,  the  water  will  only 
run  at  or  below  the  level  of  the  streets ;  and  when  in 
communication  with  the  latter,  it  may  be  drawn  in 
the  uppermost  stories  of  dwelHngs.  Here  cisterns 
are  placed,  by  which  a  supply  is  furnished  at  times 
when  the  system  is  in  communication  with  the  higher 
reservoir. 

206.  Water  is  conveyed  from  the  reservoirs  and 


PRACTICAL  MECHANICS.  187 

distributed  throughout  the  quarters  of  a  city  in  pipes 
of  cast  iron.  The  proper  dimensions  of  such  pipes 
may  be  made  a  matter  of  almost  strict  mathematical 
calculation.* 

207.  When  water  is  running  in  pipes  it  is  liable 
to  obstructions.     These  are  of  two  kinds  : 

(1.)  Water  flowing  in  a  pipe  gives  out  its  air. 
This  collects  in  the  higher  joints  of  the  pipe,  and 
forms  an  obstruction  as  effectual  as  if  it  were  a  solid 

body. 

(2.)  If  the  water  be  not  perfectly  clear,  it  will  de- 
posite  its  sediment  in  the  lower  joints  of  the  pipe,  and 
may  finally  close  it  at  these  places  altogether. 

The  first  of  these  obstructions  may  be  prevented 
by  placing  a  valve  opening  downward  at  the  high- 
est  bends  of  the  pipe.  This  is  connected  by  a  rod 
with  a  hollow  ball,  which,  being  lighter  than  water, 
keeps  the  valve  shut  so  long  as  the  pipe  is  full  of  that 
Uquid.  But  when  air  collects,  the  ball  falls  and  opens 
the  valve.  The  air  escapes,  but  is  followed  by  the 
water,  which  raises  the  ball  by  its  buoyant  force,  and 
shuts  the  valve. 

Deposites  of  sediment  may  be  removed,  wherever 
the  ground  falls  from  the  lowest  bend  of  the  pipe,  by 
means  of  a  stopcock.  This  is  opened  from  time  to 
time,  and  the  water,  flowing  rapidly  out,  carries  the 
earthy  matter  along  with  it.  In  other  cases  a  short 
pipe  is  placed  beneath  the  lower  angles  of  the  water- 
pipe,  and  communicates  with  it  in  two  places  by  lead- 
en tubes.  The  sediment,  seeking  the  lowest  level, 
will  be  formed  in  this  pipe,  which  may  be  removed 
from  time  to  time  and  emptied. 

*  See  Genieys'  Moyens  d'elever  et  de  conduire  les  eaux,  and 
Storrow  on  Water-works.  Perhaps  the  best  practical  rules  are  to 
be  found  in  Brewster's  Cyclopaedia,  article  "  Hydraulics." 


188  PRACTICAL  MECHANICS. 


IX. 

HYDRAULIC   ENGINES. 


1.  Fountain  of  Hero. 

208.  The  Fountain  of  Hero  consists  of  two  ves- 
sels, A  and  B,  the  one  placed  perpendicularly  over 
Fig.  56. 


the  other,  and  both  air-tight.     Upon  the  upper  ves- 
sel is  an  open  cistern,  C.    A  pipe,  d  d,  proceeds  from 


PRACTICAL    MECHANICS,  189 

the  cistern,  passing  through  the  upper  vessel  without 
communicating  with  it,  and  entering  the  lower  ves- 
sel, reaches  nearly  to  its  bottom.  A  second  pipe,  e  e, 
proceeds  from  the  top  of  the  lower  vessel,  and  enter- 
ing the  upper,  rises  nearly  to  its  top.  A  tliird  pipe, 
ff,  is  inserted  into  the  top  of  the  upper  vessel,  and 
reaches  within  a  small  distance  of  its  bottom  ;  this 
last  pipe  is  terminated  by  a  nozzle  or  adjutage.  The 
last  pipe  is  furnished  with  a  stopcock  at  g ;  and  there 
are  stopcocks  for  the  admission  and  discharge  of  air 
and  water. 

In  order  to  set  the  machine  in  action,  the  upper 
vessel  is  filled  with  water  nearly  to  the  level  of  the 
open  end  of  the  pipe  e  e,  and  water  is  introduced 
into  the  lower  vessel  until  the  open  end  of  the  pipe 
d  d,  is  immersed.  The  stopcocks  which  have  been 
opened  for  this  purpose,  are  now  closed,  and  wa- 
ter is  poured  into  the  cistern  C.  This  enters  and 
fills  the  pipe  d  d,  forming  a  column  of  the  whole 
height  of  the  instrument ;  by  this  the  air  contained 
in  the  upper  part  of  the  two  vessels,  and  in  the  pipe 
e  e,  is  compressed,  and  thus,  having  its  elasticity  in- 
creased, acts  upon  the  surface  of  the  water  in  the 
vessel  A  with  a  force  whose  measure  is  the  fluid 
pressure  of  the  column  in  d  d.  The  stopcock  g  is 
now  opened,  and  a  jet  of  water  is  forced  by  this  pres- 
sure out  of  the  adjutage  in  which  the  pipe  jfy  termi- 
nates. The  action  will  continue  until  the  water  is 
forced  out  of  A  to  the  level  of  the  lower  end  of  the 
pipejf/,  and  the  lower  vessel  B  is  nearly  filled  with 
water  ;  and  it  may  be  repeated  by  allowing  the  air  to 
escape  from  the  vessel  A,  and  the  excess  of  water  to 
run  out  of  the  vessel  B. 

The  Fountain  of  Hero  has  not  been  applied,  in  its 
original  form,  to  any  important  practical  purpose. 
But  the  principle  on  which  it  acts,  namely,  that  of 


190 


PRACTICAL   MECHANICS. 


compressing  a  body  of  air  by  a  column  of  fluid,  tho 
air  acting  in  its  turn  to  raise  a  second  column  of  fluid, 
has  been  advantageously  employed  in  the  instance 
we  shall  next  cite. 

2.  Machine  of  ScJiemnitz. 
209.  The  mine  of  Schemnitz,  in  Hungary,  is  situ, 
ated  in  a  mountain  which  rises  suddenly  from  a  plain. 
The  mine  was  at  first  drained  by  means  of  a  horizon- 
tal gallery  driven  in  from  the  surface  of  the  plain,' 
When  the  vein  had  been  exhausted  down  to  this 
Fig.  57. 


PRACTICAL  MECHANICS.         191 

level,  a  spring  of  water,  situated  on  the  side  of  the 
mountain,  was  niade  use  of  to  drain  it,  in  the  follow- 
ing manner  : 

A  large  airtight  vessel.  A,  was  placed  in  the  hor- 
izontal gallery.  Into  this  the  water  of  the  spring 
was  conveyed  by  the  pipe  h  h  h.  The  air  in  A  be- 
ing compressed  by  the  column  of  water  in  this  pipe, 
acted  through  the  pipe  e  e,  upon  the  surface  of  the 
water  received  from  the  bottom  of  the  mine  in  the 
vessel  C,  and  caused  it  to  rise  in  the  pipe  d  d,  and 
flow  out  at  the  level  of  the  horizontal  gallery.  The 
height  of  the  spring  above  this  gallery  was  158  feet, 
the  depth  of  the  mine  103  feet.  With  this  difference 
of  altitude,  the  quantity  of  water  raised  from  the  mine 
was  ^Vjj-  of  that  derived  from  the  spring. 

The  upper  vessel  having  been  filled  with  water, 
and  all  that  in  the  lower  having  been  forced  out,  the 
action  of  the  machine  is  renewed  by  opening  four 
stopcocks,  by  which  the  upper  vessel  is  again  filled 
with  air  and  the  lower  with  water. 

3.  Pump  of  Vialon, 

210.  The  pump  of  Vialon  is  composed  of  two 
pipes,  wound  in  opposite  directions  around  the  same 
cylinder,  at  the  top  of  which  they  are  united  in  a  sin- 
gle tube  in  the  direction  of  its  axis.  Each  tube  ter- 
minates in  a  funnel-shaped  opening  in  the  direction 
of  a  tangent  to  the  circular  base  of  the  cylinder, 
and  a  valve  opening  upward  is  placed  immediately 
above  each  funnel.  The  motion  given  to  the  instru- 
ment is  reciprocating,  and  at  each  alternation  water 
enters  by  one  of  the  funnels,  while  it  is  retained  in 
the  other  by  the  closing  of  the  valve.  The  two 
streams  will  finally  meet  in  a  vertical  pipe. 

This  instrument  is  represented  in  Fig.  58,  on  pago 
192. 


192 


PRACTICAL    MECHANICS. 

Fig.  58. 


^ 


I      -^^      I 


I       ^^^   I 


4.  Bucket  Machine, 
211.  Two  buckets,  connected  by  chains  or  ropes 
passing  over  a  pulley,  may  constitute  an  engine  for 
raising  water.  The  buckets  are  of  different  sizes, 
and  the  smaller  is  loaded  with  such  a  weight  as  to  be 
heavier  than  the  larger  when  empty,  but  the  capacity 
of  the  larger  is  so  great  that  it  shall  preponderate 
when  both  are  filled  with  water.  Both  buckets  re- 
ceive water  from  the  same  stream  and  at  the  same 
level.  The  pulley  must  be  placed  sufficiently  high 
to  allow  the  smaller  bucket  to  ascend  to  the  height 
at  which  the  raised  water  is  to  be  discharged,  and  a 
well  must  be  formed  to  permit  the  descent  of  the  lar- 


PRACTICAL  MECHANICS.         193 

ger  bucket  to  an  equal  depth  below  the  level  at  which 
both  receive  water.  When  they  have  respectively 
reached  the  highest  and  lowest  points  of  their  motion, 
the  water  is  made  to  discharge  itself  from  both  at  the 
same  instant ;  and  the  smaller  bucket  becoming  heav- 
ier, descends  and  draws  up  the  larger  bucket  until 
both  resume  their  original  position,  when  they  are 
again  filled  with  water,  and  motion  is  caused  in  the 
opposite  direction. 

The  discharge  of  the  water  from  the  buckets  may 
be  effected  in  various  ways.  The  most  ingenious  in 
principle  consists  in  taking  advantage  of  the  differ- 
ence in  the  position  of  the  centre  of  gravity*  of  a 
hollow  and  a  solid  conic  frustum.  The  buckets  are 
suspended  on  an  axis  lying  between  these  two  points, 
and  hence  are  in  stable  equilibrium  when  empty,  but 
in  one  of  tottering  equilibrium  when  full.  The 
smallest  shock  will  therefore  suffice  to  upset  them 
when  full,  but,  after  the  water  is  discharged,  they  re- 
turn to  their  primitive  position. 

212.  Two  chains  of  buckets,  passing  over  the 
same  axle,  but  having  their  openings  in  different  di- 
rections, may  be  made  to  answer  a  similar  purpose. 
The  motion  of  this  is  continuous.  The  arrangement 
will  be  understood  by  inspection  of  the  annexed 
plate.     (See  Fig.  59,  on  page  194.) 

Where  it  may  be  inconvenient  to  dig  a  well  of  suf- 
ficient depth  to  allow  the  larger  bucket  to  descend  as 
far  as  the  smaller  bucket  rises,  two  pullies  may  be 
used  of  different  diameters ;  and  the  larger  bucket, 
being  attached  to  the  smaller  of  these,  will  descend 
through  a  space  as  much  less  than  that  through 
which  the  smaller  bucket  rises  as  the  diameter  of  the 
one  pulley  is  less  than  that  of  the  other. 

*  See  Mechanics,  ^  105. 


194  PRACTICAL    MECHANICS. 

Fig.  59 


R  R  P  Q  T  is  a  wooden  frame,  through  the  top  ol  which  is 
passed  the  axle  ot  the  lantern  D  E  F  G 

IJ  B  F  E  is  a  chain  of  buckets,  which  receives  water  at  X  from 
a  reservoir.  This  chain  of  buckets  descends  into  a  well  which 
is  deep  enough  to  enable  the  buckets,  when  full  of  water,  to  set 
in  motion  those  loaded  with  the  water  which  is  to  be  raised. 

A  G  D  is  a  chain  of  buckets,  the  lower  one  of  which  is  immersed, 
as  it  revolves,  in  the  reservoir  X.  Each  bucket  has  a  spout 
near  its  top,  through  which  the  water  flows  out,  as  represented 
atw,  as  soon  as  the  bucket  begins  to  be  inclined  by  reaching  the 
curved  surface  of  the  lantern. 


PRACTICAL    MECHANICS.  195 

5.  Siphon  of  Veniuri. 

213.  When  water  is  flowing  from  a  reservoir 
through  a  cylindric  tube  of  no  great  length,  it  does 
not  fill  the  tube,  but  forms  what  is  called  the  Vena 
Conlracta,'^  The  air  contained  in  the  space  between 
the  contracted  column  of  water  and  the  sides  of  the 
tube  will  be  drawn  out  by  the  motion  of  the  liquid. 
This  action  is  analogous  to  friction,  and  is  called 
the  lateral  communication  of  motion  in  fluids.  If, 
now,  a  bent  pipe  of  smaller  size  be  inserted  into  the 
tube  through  which  the  water  is  discharged,  and  pass 
down  into  the  reservoir  of  water  beneath,  the  air 
which  this  pipe  contains  will  also  be  carried  along 
with  the  current  of  water  above  ;  and  the  pressure  of 
the  atmosphere,  acting  upon  the  surface  of  the  water 
in  the  pipe  thus  rendered  void  of  air,  will  force  it  up, 
and  cause  it  to  join  the  eflluent  stream.  The  quan- 
tity thus  raised  will  be  small,  but  there  are  cases  in 
which  it  might  be  used  to  great  advantage.  The 
limit  of  the  height  to  which  water  can  be  raised  by 
this  engine  is  the  same  as  in  the  common  pump,  say 
in  no  case  more  than  34  feet. 

A  model  of  the  Siphon  of  Venturi  is  represented 
in  Fig.  60,  on  page  196. 

6.  Hydraulic  Ram. 

214.  The  plan  of  the  Hydraulic  Ram  was  derived 
by  Mongolfier  from  the  observation  of  the  following 
fact:  If  water  running  freely  through  a  pipe  have 
its  current  suddenly  checked  by  closing  the  end 
whence  it  is  discharged,  and  there  be  a  small  hole 
on  the  upper  surface  of  the  pipe  near  this  end,  a  jet 
or  stream  of  water  will  suddenly  spout  through  this 

*  See  Mechanics,  ^  399. 


196 


PRACTICAL    MECHANICS. 

Fig.  GO. 


A  is  a  vessel  constantly  full  of  water,  which  flows  off  through 

the  pipe  C. 
6  6  is  a  siphon  introduced  into  that  part  of  the  pipe  C  in  which 

the  vena  contracta  forms. 

hole  to  a  height  much  greater  than  that  whence  the 
velocity  of  the  water  in  the  pipe  is  derived.  This 
jet  will  only  continue  for  a  short  time,  and  will  grad- 
ually decrease  in  height  until  that  amounts  to  no 
more  than  the  effective  head  of  water.  By  opening 
the  pipe  again,  and  thus  permitting  the  water  in  it  to 
resume  its  original  velocity,  the  operation  may  be 
repeated.  The  cause  of  the  rise  of  the  water  from 
the  small  hole  is,  that  the  velocity  being  suddenly 
checked,  the  whole  quantity  of  motion  of  the  water 
in  the  pipe  is  exerted  to  force  a  part  through  the 
small  opening  ;  and  abstracting  from  friction  and 
other  resistances,  the  height  of  the  jet  will  bear  to  the 
effective  head  of  the  water  in  the  pipe,  the  relation 
that  the  area  of  the  latter  bears  to  that  of  the  former. 


PRACTICAL    MECHANICS.  197 

n  order  to  render  this  principle  efficient  in  prac- 
tice, it  is  necessary  to  cause  the  water  to  rise  in 
a  pipe  instead  of  a  jet  in  the  open  air ;  to  place  a 
valve  on  this  rising  pipe,  in  order  to  prevent  the  re- 
turn of  the  water  it  contains,  when  the  water  in  the 
inain  pipe  resumes  its  flow ;  and  to  adapt  a  self-act- 
ing valve  to  the  main  pipe,  by  which  it  maybe  open- 
ed and  shut  alternately.  The  latter  object  is  attained 
by  the  application  of  a  simple  principle.  Water, 
when  in  motion,  is  capable  of  carrying  along  with  it 
bodies  of  greater  density  than  itself^  and  of  sizes 
having  relation  to  their  own  density  and  the  velocity 
of  the  water.  Thus  large  and  heavy  masses  of  rock 
are  swept  along  by  torrents  ;  rounded  stones,  peb- 
bles, and  gravel  by  streams  of  less  intensity.  On 
the  other  hand,  as  soon  as  the  velocity  is  checked, 
such  substances,  are  deposited,  in  consequence  of  their 
superior  density,  which  causes  them  to  sink.  A  valve 
constructed  upon  this  principle  will  not  open  imme- 
diately after  it  closes,  but  will  remain  shut  an  appre- 
ciable time,  because  a  body  in  motion  does  not  im- 
mediately lose  all  its  velocity,  but  must  pass  through 
every  intermediate  rate  of  motion  from  its  maximum 
to  0  ;  and  the  valve  does  not  forthwith  close,  because 
it  requires  a  definite  time  for  a  body  at  rest  to  resume 
its  previous  velocity.  The  latter  part  of  the  princi- 
ple may  be  illustrated  by  a  number  of  facts :  a  can- 
non ball  will  strike  a  mark  as  certainly  if  fired  from 
a  piece  suspended  freely  on  a  pivot,  as  if  fired  from 
one  firmly  fastened  down,  because  there  is  not  time 
for  the  motion  to  be  communicated  throughout  the 
whole  body  of  the  piece  before  the  ball  leaves  the 
muzzle  ;  a  person  who  passes  rapidly  over  thin  ice 
or  a  weak  board,  may  do  so  without  breaking  them, 
while,  if  he  move  slowly,  they  are  infallibly  ruptured  ; 
a  rope  tied  to  a  shell  which  is  fired  from  a  mortar 
16 


198  PRACTICAL    MECHANICS. 

is  broken,  however  freely  it  may  bo  coiled,  because 
the  motion  has  not  had  time  for  its  transmission 
throughout  the  whole  length. 

It  was,  moreover,  found  that  the  sudden  action,  by 
which  the  jet  is  caused  to  rise,  was  apt  to  burst  the 
rising  pipe  or  force  water  through  its  joints.  To 
prevent  this,  an  air  vessel  was  placed  on  this  pipe, 
the  air  contained  in  which  should  act  as  a  spring  to 
regulate  the  action,  and  cause  a  continual,  although 
not  a  steady,  flow  of  the  water  in  its  ascent. 

These  facts  and  principles  being  premised,  the 
structure  and  action  of  the  instrument  may  be  under- 
stood.    (See  Fig.  61,  on  page  199.) 

The  Hydraulic  Ram  is  Hable  to  difficulties  in  its 
action,  in  consequence  of  the  tendency  which  air  has 
to  mix  with  water.  That  which  is  contained  in  the 
airvessel  may  therefore  be  finally  exhausted,  and 
the  action  of  that  part  of  the  engine  may  thus  cease. 
To  remedy  this  defect,  valves  have  been  planned  by 
which  new  suppHes  of  air  may  be  obtained  from  the 
atmosphere. 

7.  Pumps, 
215.  The  principle  on  which  the  common  pump 
acts  forms  a  part  of  the  theory  of  Mechanics.*  It 
is  sufficient  for  our  purpose  to  recollect  that  water 
is  forced  by  the  pressure  of  the  air  into  a  pipe  which 
would  otherwise  become  a  vacuum,  in  consequence 
of  the  alternating  action  of  a  piston  furnished  with 
a  valve  opening  upward.  The  reflux  of  water  is 
prevented  by  a  fixed  valve,  also  opening  upward. 
Pumps  of  the  common  kind,  therefore,  rather  differ 
from  each  other  in  the  material  of  which  they  are 
constructed  and  the  form  of  the  valves,  than  in  any 
other  respect. 

*  See  Mechanics,  ^  358. 


PRACTICAL    MECHANICS. 

Fig.  61. 


199 


B,  a  pipe  through  v/hich  water  Js  flowing  from  a  millpond  or  res- 
ervoir. This  is  bent  upward  at  right  angles,  and  the  water,  if 
unobstructed,  would  be  discharged  at  C.  In  the  bend  of  the  pipe 
is  a  valve,  D,  of  a  spherical  figure,  of  sach  weight  as  to  sink  in 
water,  but  not  so  dense  that  it  will  not  rise  when  the  water  in  the 
pipe  B  has  acquired  all  the  velocity  which  is  due  to  its  effective  head.^ 

abed  is  a  short  pipe  adapted  to  the  pipe  B,  and  directed  »]p- 
ward.  In  this  pipe  is  situated  a  valve,  E,  also  of  a  spherical  fig- 
ure. This  alternates  in  its  motion  with  the  valve  D,  closing  the 
passage  a  b  when  C  is  open,  and  opening  when  it  shuts. 

F,  air  vessel. 

G  H,  ascending  pipe,  in  which  the  'vater  is  raised  by  the  tension 
of  the  air  compressed  in  the  airvessel  F. 


200  PRACTICAL    MECHANICS* 

(1.)  The  valves  of  the  cheapest  form  of  the  com- 
mon pump,  represented  in  the  figure,  are  no  more 
than  a  circular  plate  of  leather,  on  a  part  of  the 
circumference  of  which  is  a  rectangular  projection, 
which  is  nailed  down  to  a  collar  in  the  barrel  or  in 
the  piston,  and  which  thus  serves  as  a  hinge.  The 
leather  is  stiffened  by  nailing  a  piece  of  wood  on  the 
lower  side.  India  rubber,  or  cloth  prepared  with 
that  substance,  has  been  found  to  answer  better. 

Fig.  62. 


(2.)  A  circular  plate  of  leather  may  be  nailed  ia 
the  direction  of  one  of  its  diameters  to  a  bar  which 
crosses  the  barrel  of  the  pump,  or  the  hollow  of  the 
piston.  This  valve,  from  a  resemblance  in  its  mo- 
tion to  that  of  wings,  is  called  the  butterfly  valve. 
It  may  also  be  executed  in  metal,  in  which  case  the 
two  parts  have  the  shape  of  a  portion  of  a  circle 
somewhat  less  than  a  semicircle,  and  are  connected 


PRACTICAL    MECHANICS. 


201 


with  the  bar  by  a  hinge.  This  form  of  valve  has 
already  been  spoken  of  as  that  used  in  the  airpump 
of  a  steam-engine. 

(3.)  When  the  body  of  the  pump  is  a  square,  a 
valve,  called,  from  its  figure,  pyramidal,  is  sometimes 
used.  This  has  for  its  basis  a  frame  having  the  form 
of  the  four  edges  of  a  regular  pyramid.  The  move- 
able parts  of  the  valve  are  four  equilateral  triangles 
of  leather,  stiffened  by  wood,  and  nailed  at  the  base 
of  the  pyramid. 

(4.)  The  triangular  valve  is  appHed  to  a  pump 
whose  body  is  also  square.  The  seat  of  the  valve 
is  composed  of  two  equilateral  triangles,  vvhich,  when 
introduced  into  a  square,  lie  in  an  inclined  position. 
The  leather  plate  has  the  same  figure,  and  is  nailed 
to  a  diagonal  bar. 

(5.)  The  conical  valve  (Fig.  63)  is  always  made 
of  metal,  and  has  the  shape  of  the  frustum  of  a  cone, 
Fig.  63. 


adapting  itself  to  a  seat  of  the  same  figure.  This 
valve  requires  to  be  guided  by  a  rod  passing  through 
a  bar  fixed  in  the  body  of  the  pump  or  in  the  move- 


202 


PRACTICAL    MECHANICS. 


able  piston.  This  valve,  which  is  among  the  best 
when  the  water  is  clean,  is  liable  to  be  choked  by  the 
entrance  of  solid  matter. 

(6.)  The  spherical  valve  is  a  hollow  sphere  of 
metal,  which  applies  itself  to  a  seat  having  the  figure 
of  a  zone  of  a  sphere.  This  valve  is  represented 
in  Fig.  64. 

Fig.  64. 


A  B  C  D,  Barrel  of  the  pump.  " 

s,  Valve  seat  of  the  figure  of  a  zone  of  a  hollow  sphere. 

S,  Valve  of  the  figure  of  a  sphere. 

a  b  c,  Bar  extending  across  the  pump  barrel,   to  bear  flexible 

strips  of  metal  which  keep  the  motion  of  the  valve  within 

proper  limits. 
hdyb  e,  b  f^  Flexible  strips  of  metal. 

This  valve  is  less  liable  to  choke  than  the  conical 
valve,  and  may  be  rendered  almost  incapable  of  be- 
inp:  choked  by  adapting  a  long  rod  to  it,  and  loading 


PRACTICAL    MECHANICS.  203 

the  end  of  the  rod  with  a  weight.  With  this  addi- 
tion the  cage  becomes  unnecessary,  and  it  is  now 
called  the  pendulum  valve. 

Such  are  a  few  of  the  many  forms  which  have 
been  proposed  for  the  valves  of  pumps. 

216.  The  common  pump  is  an  apparatus  of  great 
convenience,  and,  wherever  the  quantity  of  water  re- 
quired is  such  that  the  expenditure  of  the  moving 
power  may  be  disregarded,  is,  perhaps,  the  most  use- 
ful of  all  hydraulic  engines.  Even  in  such  instances 
it  is  limited  in  its  use  by  the  fact  that  the  rise  of  the 
water  is  due  to  atmospheric  pressure,  which,  unless 
the  materials  and  workmanship  are  superior  to  those 
usually  employed,  cannot  be  relied  upon  if  the  height 
of  the  lower  valve  above  the  water  to  be  raised  ex- 
ceeds 28  feet. 

217.  When  the  pump  is  to  be  kept  in  continued 
action,  the  quantity  of  force  which  will  be  required 
to  move  it  becomes  an  important  object,  and  the 
pump  is  a  disadvantageous  application  of  the  force. 
It  is  stated  by  Hachette  that  the  measure  of  its  work 
is  not  equal  to  more  than  one  tenth  of  that  of  the 
prime  mover. 

When  a  common  pump  is  worked  by  men,  the  ap- 
plication of  the  force  is  still  more  disadvantageous ; 
for  the  particular  manner  in  which  a  man  works  the 
brake  of  a  pump  constitutes  a  labour  which  exhausts 
more  rapidly  than  almost  any  other.  There  are, 
notwithstanding,  cases  in  which  no  adequate  substi- 
tute  has  been  introduced  into  general  use. 

The  great  friction  which  attends  the  motion  of  the 
piston  of  a  pump,  when  packed  in  such  manner  as  to 
be  air  tight,  is  one  of  the  causes  of  the  loss  of  force. 
It  has  been  attempted  to  obviate  this  by  enlarging 
the  part  of  the  pump  in  which  the  piston  acts,  and 


204 


PRACTICAL    MECHANICS. 


connecting  the  piston  to  the  seat  of  the  lower  valve 
by  a  hollow  vessel  of  leather.  The  best  form  of  this 
is  made  up  of  a  number  of  rings  of  leather  united  at 
their  outer  edges.  We  annex  a  draught  of  such  a 
pump  taken  from  the  work  of  Hachette. 
Fig.  65. 


This  has  recently  been  published  in  this  country 
as  a  new  invention. 

Martin's  ship  pump  is  somewhat  similar.  The 
pompe  des  pretres  has  its  moveable  valve  situated  in 
a  loose  diaphragm  of  leather,  placed  in  an  enlarge- 
ment  of  the  body  of  the  pump. 

218.  It  is  a  common  mistake  to  suppose  that,  as 
the  moving  power  in  the  common  pump  is  the  pres- 
sure of  the  atmosphere,  there  is  but  little  force  em- 
ployed to  move  it.  But  this  pressure  is  no  more 
than  a  machine  interposed  between  the  prime  mover 
and  the  water  to  be  raised,  and  does  not  act  of  itself. 


PRACTICAL    MECHANICS.  205 

In  order  to  bring  the  pump  into  that  state  in  which 
water  will  flow  at  a  single  stroke  of  the  piston,  as 
much  force  must  have  been  previously  applied,  in 
addition  to  that  intended  to  overcome  the  Motion,  as 
would  raise  the  water  to  the  level  of  the  lower  valve 
in  any  other  way. 

219.  The  common  pump  being  limited  in  its  ac- 
tion to  heights  which,  under  the  most  advantageous 
circumstances,  do  not  exceed  34  feet,  pumps  of  an- 
other description,  called  forcing  or  lifting,  are  used 
where  the  height  to  which  the  water  is  to  be  raised 
exceeds  that  limit.  The  latter  of  these  is  rarely 
used.  The  former  differs  from  the  common  pump 
in  having  a  solid  piston.  A  pipe  proceeds  from  the 
body  of  the  pump,  at  a  point  just  above  the  fixed 
or  lower  valve.  This  pipe  is  furnished  with  a  valve 
opening  upward.  The  action  of  the  piston  in  rising 
is  similar  to  that  of  the  common  pump  ;  but  on  its 
descent  it  forces,  first,  the  air  contained  in  the  body 
of  the  pump,  and  subsequently  the  water  raised  by 
its  previous  action,  through  the  last-named  valve  into 
the  pipe  we  have  described.  The  previous  action 
has  the  same  limit  as  the  common  pump,  say,  under 
ordinary  circumstances,  28  feet ;  but  the  latter  ac- 
tion has  no  limit  except  the  strength  of  the  materials 
of  which  the  pump  is  constructed,  and  the  intensity 
of  the  agent  employed  to  work  it. 

220.  In  the  best  form  of  forcing-pump,  the  piston 
does  not  work  against  the  sides  of  its  body,  but  has 
the  form  of  a  plunger,  which  is  passed  through  a  col- 
lar  that  closes  the  upper  part  of  the  body  of  the  pump. 
With  this  form  of  piston,  the  friction  of  the  pump  is 
considerably  lessened. 

A  pump  of  this  form  is  represented  in  Fig.  66,  on 
the  following  page. 

17 


206 


PRACTICAL    MECHANICS. 
Fig.  66. 


S  S.  Body  of  the  pump. 
P  M.  Solid  plunger. 
H.  Pump-rod. 

E.  Suction  valve. 
D.  Force  Valve. 

R.  Rising  pipe,  through  which 
the  water  is  forced  up  when 
the  plunger  descends. 

F.  Pipe  through  which  the  wa- 
ter rises  by  the  pressure  of  the 


atmosphere  when  the  plunger 
is  drawn  up. 

X  Z.  Collar  enclosing  a  pack- 
ing of  oiled  leather,  by  which 
the  joint  between  the  barrel 
of  the  pump  and  the  plunger 
is  rendered  air-tight. 

s  s.  Channel  by  which  any  air 
that  may  lodge  in  the  pump 
can  be  permitted  to  escaoe. 


PRACTICAL    MECHANICS.  207 

A,i  the  friction  of  a  pump  of  given  dimensions  is  a 
constant  quantity,  the  force  required  to  overcome 
this  resistaiice  in  a  force-pump,  raising  water  28  feet 
by  the  pressure  of  the  atmosphere,  and  28  feet  by  the 
forcing  action,  is  no  more  than  in  a  common  pump ; 
and  with  every  increase  in  the  height  to  which  tiie 
water  is  raised,  the  proportion  between  tlie  useful  ac. 
tion  of  the  prime  mover  and  the  loss  by  friction  will 
be  lessened.  A  force-pump,  therefore,  which  raises 
water  to  a  great  height,  is  a  much  more  advantage- 
ous application  of  a  prime  mover  than  the  common 
pump.* 

The  action  of  the  force-pump  is  regulated  by  the 
addition  of  an  airvessel.  This  is  placed  upon  the 
pipe  through  which  the  water  is  raised.     The  rea- 

♦  Pumps  are  worked  to  greater  advantage  by  the  steam-engine 
than  by  any  other  agent.  In  fact,  tiie  reciprocating  motion  ot  the 
engine,  which  corresponds  identically  with  that  of  a  pump,  was 
originally  contrived  for  this  very  object,  and  the  successive  changes 
in  the  structure  of  the  engine  have  taken  the.  character  merely  of 
improvements.  In  the  use  of  pumps  of  large  size  driven  by  a 
steam-engine,  it  has  been  found  that  the  stroke  of  the  piston  must 
not  exceed  8  feet.  The  proper  velocity  in  feet  per  minute  will  be 
found  by  multiplying  the  square  root  of  the  length  of  stroke  by  the 
constant  number  98.  The  quantity  of  water  in  cubic  feet  per  min- 
ute is  found  by  the  continued  multiplication  of  half  the  velocity  of 
the  piston  in  feet,  the  square  of  the  velocity  in  inches,  and  the 
constant  fraction  0.00518. 

The  diameter  of  the  piston  of  the  pump  in  inches  is  found  by 
the  formula 

d=^2.l5  W; 

in  which  W  is  the  quantity  of  water  to  be  discharged  in  cubic  feet. 
The  corresponding  diameter  of  the  piston  is  found  by  the  for- 
mula 


J)=^ 


/7332  W  h\ 


in  which  p  is  the  average  pressure  on  the  piston,  and  H  the  effect- 
ive height  to  which  the  water  is  t-o  be  raised.  H  is  found  by  add- 
ing to  the  real  height  1|  feet  for  every  separate  lift,  and  one  twen 
tieth  of  the  length  of  the  pipes  of  which  the  pump  is  composed. 


208  PRACTICAL  MECHANICS. 

sons  for  using  an  airvessel  in  the  hydraulic  ram  are 
applicable  in  this  instance  also. 

221.  The  common  fire-engine  is  composed  of  two 
force-pumps,  which  throw  water  into  a  single  air- 
vessel.  The  two  pumps  are  connected  by  a  brake 
or  lever,  having  a  fulcrum  lying  between  them  ;  they 
therefore  act  alternately,  and  the  action  of  the  air- 
vessel  produces  a  constant  stream. 

Fig.  67  is  a  section  of  a  fire-engine,  exhibiting 
one  of  the  pumps. 

Fig.  67. 


11 


PRACTICAL    MECHANICS. 


209 


L  is  the  piston  of  the  pump,  the  rod  of  which  is  attached  to  a 
circular  segment,  K  N. 

The  lower  or  suction  valve  is  opposite  F. 

The  valve  G  forms  a  communication  with  the  large  airvessel, 
whence  the  water  is  forced,  by  the  elasticity  of  the  compressed  air, 
through  the  pipe  V, 

At  T  IS  a  screw  to  which  a  hose  or  pipe  may  be  applied. 

At  D  is  a  screw  to  wliich  a  hose  may  be  adapted,  through  which 
water  is  forced  by  the  pressure  of  the  atmosphere  to  the  valve  E. 
The  pipe  which  conveys  the  water  from  the  hose  to  the  valve 
may  be  closed  by  a  stopcock,  which  is  turned  by  the  lever  Y  X ; 
and  when  this  communication  is  Pj     ^.^^ 

closed,  one  is  at  the  same  time  fc-    -■ 

opened  with  a  cistern,  which  is 
formed  by  a  box  enclosing  the 
pumps  and  valves.  This  cistern 
may  be  filled  from  a  grated  open- 
ing above  the  inclined  side  Z. 

222.  The  defects  of  the 
common  pump  have  led 
to  the  invention  of  rota- 
ry pumps.  In  these  the 
valves  work  in  a  ring  ;  the 
water  enters  on  one  side 
of  tlie  ring,  and  is  forced 
out  at  a  point  nearly  oppo- 
site«  A  rotary  pump  may 
act  as  a  common  or  for- 
cing-pump, according  to 
its  position.  Of  rotary 
pumps  there  are  several 
varieties,  but  the  friction 
in  any  of  them  is  far  less 
than  in  either  the  common 
or  forcing  pump.  It  would, 
in  consequence,  be  a  great 
saving  of  labour  could  the 
rotary  pump  and  appara- 
tus for  working  it  be  ap- 
plied to  a  carriage,  in  or- 
der to  serve  as  a  fire-en- 


210 


PRACTICAL    MECHANICS. 


gine.    Sections  of  two  rotary  pumps  are  represented 
in  Fig.  68,  on  page  209. 

Among  other  forms  which  have  been  proposed  to 
remedy  the  defects  of  the  common  and  forcing  pumps, 
are  that  of  Vera,  and  the  centrifugal  pump. 

8.  Pump  of  Vera. 

223.  The  pump  of  Vera  is  composed  of  an  end. 
less  rope  stretched  in  a  vertical  position  between  two 
pulleys.  The  upper  pulley  is  made  to  revolve  by  a 
winch,  or  by  bands  passing  over  wheels,  so  arranged 
as  to  increase  the  velocity  of  the  motion,  and  the 
rope  will  revolve  with  the  pulley.  The  lower  pulley 
is  situated  in  the  reservoir  whence  water  is  to  be 
raised,  and  the  rope,  in  turning  under  this  pulley,  be- 
Fig.  69. 


;omes  charged  with  the  fluid,  which   it  carries  up 
with  it  until  it  is  in  the  act  of  passing  over  the  upper 


PRACTICAL    MECHANICS. 


211 


pulley.  In  this  change  in  the  direction  of  the  motion 
of  the  rope  the  water  does  not  immediately  partici- 
pate, and  is  therefore  thrown  off.  The  water  thrown 
off  is  received  in  an  appropriate  vessel,  whence  it  is 
discharged  hy  a  spout. 

The  efficient  action  of  this  pump  depends  upon 
the  degree  of  tension  of  the  rope.  If  this  tension  be 
too  great,  the  friction  will  require  too  great  a  part  of 
the  moving  power  ;  while,  if  the  tension  be  too  small, 
the  rope  will  slide.  The  greater  velocity  of  the  rope, 
the  more  of  the  water  which  the  rope  takes  up  will 
be  carried  with  it  to  the  upper  pulley.  Ropes  made 
of  hair  are  found  to  carry  more  water  than  those 
of  hemp. 

9.   Centrifugal  Pump. 

224.  In  the  centrifugal  pump  a  number  of  pipes 

are  arranged  on  the  surface  of  a  truncated  cone, 

WJose  least  base  is  lowest.    The  upper  part  of  each 

Fig.  70. 


212  PRACTICAL    MECHANICS. 

pipe  is  bent  outward,  and  then  turns  downward  for  b 
short  distance.  When  a  rapid  motion  of  revoiutior 
is  given  to  the  cone,  the  centrifugal  force  will  cause 
any  water  with  which  the  pipes  are  loaded  to  be  dis- 
charged at  their  upper  orifice ;  and  if  the  lower  end 
be  immersed  in  water,  a  column  of  that  liquid  will 
continue  to  flow  upward  through  the  pipe. 

This  kind  of  pump  is  principally  worthy  of  notice 
as  an  illustration  of  the  mistakes  which  may  be  made 
in  mechanics  even  by  intelligent  men  ;  for  it  has  been 
maintained  by  some,  that  as  the  force  in  comphance 
with  which  the  water  is  discharged  is  a  consequence 
of  the  rotation,  more  water  might  be  raised  than 
would  be  equivalent  to  the  energy  of  the  prime  mo- 
ver. Those  who  reasoned  in  this  way  forgot  that 
the  centrifugal  force  is,  in  fact,  the  force  to  which  the 
revolution  is  due,  and  derived  immediately  from  the 
prime  mover,  whose  intensity  it  therefore  can  never 
exceed,  but  will  be  always  as  mucli  less  as  is  due  to 
the  friction  and  other  resistances. 

10.   Chain-'pumjp, 

225.  The  chain-pump  is  composed  of  an  endless 
rope  or  chain,  to  which  a  number  of  buckets  are  at- 
tached. This  apparatus  is  passed  over  a  fixed  pul- 
ley, which  is  caused  to  revolve  by  the  prime  mover, 
and  carries  the  chain  of  buckets  with  it.  The  buck- 
ets may  be  open  vessels  of  any  form,  or  they  may  be 
merely  flat  boards,  through  the  middle  of  which  the 
rope  or  chain  is  passed.  In  the  former  case,  the 
pulley  is  either  round  or  polygonal,  having  sides 
equal  in  length  to  the  links  of  the  chain.  In  the  lat- 
ter case,  the  pulley  is  a  mere  skeleton  composed  of 
six  radii,  between  which  the  buckets  fall,  and  which 
are  forked  at  their  extremities  in  order  to  take  hold 
of  the  chain.     In  this  form  the  ascending  branch  of 


PRACTICAL   MECHANICS. 


213 


the  chain  passes  into  a  barrel,  which  its  buckets  near- 
ly  fill  at  the  lower  end,  but  which  is  enlarged  towards 
its  upper  end,  in  order  that  there  may  be  no  risk  of 
the  buckets  touching  it.  The  lower  end  of  the  barrel 
is  immersed  in  water ;  and,  when  the  chain  is  set  in 
motion  by  turning  the  pulley,  water  will  be  forced 
into  the  barrel  by  each  bucket,  the  greater  part  of 
which  will  be  carried  through  the  barrel  and  dis- 
charged at  its  upper  end.  A  chain-pump  of  this 
form  is  represented  Fig.  71. 
Fig.  71. 


C  C,  Descending  buckets. 


F  D.  Ascending  buckets. 


214        PRACTICAL  MECHANICS. 

E.  Pump  barrel  through  which  the  ascending  buckets  rise ;  the 
lower  bucket  entering  at  a,  and  the  upper  bucket  discharging  the 
water  taken  up  at  b  through  the  spout  H. 

m  n.  Level  of  the  water  in  the  reservoir. 

When  a  chain-pump  is  worked  by  the  force  of 
men  acting  upon  two  v/inches  on  opposite  sides  of 
the  axis,  the  quantity  of  water  raised  has  been  found 
equal  to  three  fourths  of  the  measure  of  human  force 
given  in  §  31. 

226.  It  is  sometimes  necessary  to  place  a  chain- 
pamp  in  an  inclined  position,  in  order  that  water  may 
be  discharged  over  a  dike  or  bank.  In  this  case, 
the  rising  branch  of  the  chain-pump  lies  in  an  open 
inclined  channel,  and  the  lower  face  of  the  buckets 
touches  the  bottom  of  this  channel.  A  greater  fric- 
tion than  in  the  vertical  form  is  the  consequence,  and 
the  work  performed  is  diminished  to  seven  tenths 
of  the  measure  of  the  moving  power,  when  men  or 
animals  are  employed. 

227.  An  inclined  chain-pump,  formed  of  strong 
scoops  attached  to  a  chain,  and  passing  over  a  polyg- 
onal pulley,  is  now  used  in  the  operation  called 
dredging,  or  the  removal  of  loose  matter  from  the 
bed  of  streams.  This  may  be  set  in  motion  by  a 
steam-engine,  or  by  horses.  The  first  application  of 
the  chain-pump  to  this  purpose  was  made  by  Evans, 
:n  1801,  on  the  Delaware.  This  experiment  is  the 
jnore  remarkable,  as  it  was  accompanied  with  the 
successful  propulsion  of  the  vessel  on  which  the  ma- 
chine  was  erected,  by  means  of  the  steam-engine, 
not  only  through  the  water,  but  •  along  the  ground 
which  intervened  between  his  workshops  and  the 
river. 

A  dredging-machine  is  represented  Fig.  72,  oppo- 
site. 


PRACTICAL    MECHANICS. 


215 


AAA  Section  of  the  vessel  on  which  the  machine  is  Placed, 
having  a  well  in  the  space  d  k,  through  which  the  chain  ot  bucK 
ets  works. 


216         PRACTICAL  MECHANICS. 

D  D.  Chain  of  buckets. 

C  B  H.  inclined  plane  furnished  with  rollers,  over  which  the 
loaded  buckets  rise. 

F  H.  Pulleys  around  which  the  chain  of  buckets  turns. 

kl,  h  g,  &c.  Shafts,  wheels,  and  pinions  through  which  the 
prime  mover  is  transmitted  to  the  pulley  F. 

I.  Spout  through  which  the  matter  raised  by  the  machine  is 
discharged  into  a  barge  J. 

K.  Pinion  and  wheel,  connected  by  a  rope  and  pulley  E  with  the 
pulley  H,  by  means  of  which  the  lower  end  of  the  chain  of  buck- 
ets is  set  to  the  desired  depth. 

d.  Level  of  the  surface  of  the  v/ater. 

11.  Screw  of  Arcldmedes. 

228.  The  screw  of  Archimedes  may  be  conceived 
to  be  formed  by  wrapping  a  flexible  pipe  around  a 
solid  cylinder,  in  the  form  of  a  screw.  This  cyUnder 
is  supported  upon  two  gudgeons,  in  such  manner  that 
its  axis  shall  be  more  inclined  to  the  horizon  than 
the  thread  of  the  screw  is  to  this  axis.  If,  therefore, 
the  lower  end  of  the  apparatus  be  immersed  in  water, 
that  fluid  will  enter  at  the  lower  opening  of  the  pipe. 
If  the  screw  be  turned  on  its  axis  in  the  direction 
by  which  a  screw  is  forced  downward,  the  water 
which  has  entered  will  move  along  the  inclination  of 
the  thread  of  the  screw,  and  when  the  number  of 
revolutions  shall  equal  the  number  of  convolutions  of 
the  pipe  on  the  cylinder,  it  will  have  risen  to  the  up- 
per end  of  the  pipe,  where  it  will  be  discharged.  At 
each  revolution  an  additional  quantity  of  water  enters 
the  screw. 

229.  The  screw  of  Archimedes,  in  its  more  usual 
form,  is  made  by  enclosing  a  spiral  surface  be- 
tween a  solid  axle  and  a  hollow  cylinder.  This  has 
its  maximum  effect  when  the  lower  end  of  the  cylin- 
der is  immersed  to  its  horizontal  diameter.  Half  of 
a  convolution  of  the  spiral  will  be  thus  filled  with 
water  at  each  revolution  of  the  axle.  When  the 
water  in  the  reservoir  has  a  varying  level,  it  has 
been  found  better  to  omit  the  hollow  cylinder,  ai^'^ 


PRACTICAL    MECHANICS.  217 

Fig.  73. 


A  B.  Level  of  the  water  in  the  reservoir. 

P.  Winch  or  handle  by  which  the  screw  is  turned. 

cause  the  screw  to  work  in  an  open  inclined  channel, 
having  the  shape  of  the  half  of  a  cylinder.  The 
quantity  of  work  performed  by  men  and  animals  with 
this  machine  is  about  three  fourths  of  the  measure 
of  the  moving  power. 

12.  Flash  Wheel. 
230.  The    engine  called   the    flash  or  fen  wheel 
may  be  considered  as  an  undershot  wheel  having  its 
motion  reversed,  and  thus,  under  the  action  of  some 


218 


PRACTICAL   MECHANICS. 


prime  mover,  raising  a  current  of  water  up  a  small 
height.  It  has  the  advantage  of  great  simpHcity,  and 
is  attended  with  Httle  friction.  In  its  best  form  it  is 
placed  in  a  channel  which  is  nearly  filled  by  its 
buckets,  like  the  modification  of  a  breast-wheel  rep- 
resented in  Fig.  17 ;  and  its  buckets  are  so  placed 
as  to  be  vertical  at  the  time  the  water  is  discharged 
from  them  into  the  horizontal  channel  which  is  to 
carry  it  off.  This  apparatus  has  been  for  ages  ad. 
vantageously  used  in  Holland,  for  draining  the  sur- 
face-water off  embanked  meadows.  In  that  country 
it  is  moved  altogether  by  windmills.  In  England 
it  has  recently  been  moved  by  steam-engines,  and,  in 
late  experiments,  an  engine  of  80  horse  power  raised 
9840  tons,  6  feet  7i  inches  in  the  space  of  an  hour. 
This  is  equivalent  to  29,164  lbs.  raised  one  foot  per 
minute  by  each  horse  power ;  the  consumption  of 
coals  during  the  hour  was  10  bushels.     The  form  and 


Fig.  74. 


PRACfLCAL  MECHANICS.  219 

character  of  this  flash  wheel  may  be  understood  from 
Fig.  74. 

231.  We  have  given  but  a  small  selection  of  the 
almost  innumerable  forms  of  engines  for  raising  wa- 
ter. Among  those  which  remain  are  the  Tympa- 
num and  Noria,  of  ancient  celebrity.  Since  the  in- 
troduction of  the  use  of  steam,  there  has  been  but  lit- 
tle need  of  directing  attention  to  these  instruments, 
once  of  great  note. 

13.  Hydraulic  Press. 

232.  The  hydraulic  press  depends  upon  the  prin- 
ciple in  the  mechanics  of  fluids  which  is  called  th; 
hydrostatic  paradox.  It  thus  happens,  that  the  pres- 
sure of  any  column  of  fluid,  however  small,  may  be 
made  to  counterbalance  that  of  any  other  column, 
however  great.  In  conformity  with  this  principle,  if 
there  be  a  communication  between  two  columns  of 
the  same  fluid,  whatever  pressure  may  be  exerted 
upon  the  one  will  be  transmitted  to  the  other,  in  .a 
ratio  proportioned  to  the  respective  area  of  the  two 
columns. 

In  the  hydraulic  press,  a  small  pump,  with  a  solid 
plunger,  is  made  to  force  water  into  a  cylinder  of 
much  greater  diameter  than  itself,  and  in  which  a 
solid  plunger  is  also  placed.  The  quantity  of  pres- 
sure which  is  thus  transmitted  to  the  latter,  is  as 
much  greater  than  that  which  the  prime  mover  ex- 
erts upon  the  piston  of  the  former,  as  the  area  of  the 
former  is  greater  than  that  of  the  latter.  When  this 
instrument  is  applied  as  a  press,  it  has  the  form  of 
Fig.  75,  on  page  220. 


233.  The  increase  of  the  intensity  of  force  by 
means  of  the  water-press  exceeds  any  that  can  be 
produced  in  other  ways,  except  by  a  complex  ar- 


220  PRACTICAL    MECHANICS. 

Fig.  75. 


rangement  of  machiriery.  It  has  therefore  been  ap- 
plied to  many  other  purposes  than  mere  pressure. 
Thus,  it  is  employed  to  prove  steam-boilers,  water- 
pipes,  and  cannon  ;  to  tear  up  the  roots  of  trees  ;  to 
draw  piles,  after  the  purpose  for  which  they  were 
driven  has  been  accomplished.  In  proving  cannon, 
it  has  been  found  not  only  to  have  the  effect  of  show- 
ing any  flaws  which  may  exist  in  the  casting,  but  also 
to  remedy  them.  It  has,  in  fact,  been  observed,  that 
after  a  cannon  has  been  made  to  leak  under  the  ac- 
tion of  the  water-press,  it  has,  on  renewing  the  same 
proof  after  an  interval  of  some  days,  become  tight. 
The  explanation  of  this  is  to  be  found  in  the  action 
of  the  water  forced  in  among  the  crystalline  particles 
of  the  iron,  to  rust  their  surfaces,  and  thus  unite  them 
by  a  ferruginous  cement. 

The  hydraulic  dock,  which  has  been  mentioned  in 
§  197,  is  a  beautiful  application  of  this  machine.    A 


PRACTICAL    MECHANICS.  221 

platform  of  sufficient  size  to  receive  the  ship,  and 
strong  enough  to  bear  its  weight,  is  suspended  by  a 
number  of  chains,  which  are  attached  on  each  side 
of  the  platform  to  a  horizontal  beam.  Each  chain 
touches  a  pulley,  over  which  it  is  bent  when  the  beam 
is  drawn  forward,  and  the  chains  drawn  by  the  beam 
over  these  pulleys  lift  the  platform.  The  two  beams 
are  drawn  forward  by  the  great  piston  of  a  hydraulic 
press,  which  works  in  a  horizontal  cylinder.  The 
force-pump  of  this  press  may  be  worked  by  steam. 
By  means  of  the  press  a  small  force  has  its  intensity 
so  far  increased  as  to  lift  the  largest  vessel.  In  the 
port  of  New- York,  vessels  of  1000  tons  burden  have 
been  lifted  by  this  apparatus. 


In  the  above  list  we  have  included  some  of  the 
more  important  or  interesting  hydraulic  engines. 
The  number  which  have  been  proposed  or  actually 
introduced  is  so  great  that  even  to  name  them  would 
exceed  our  limits. 

18 


222  PRACTICAX    MECHANICS. 

X. 

EQUILIBRIUM   AND   MOTION    OF   VESSELS. 

234.  However  irregular  may  be  the  figure  of 
vessels  intended  for  navigation,  the  theory  of  their 
equilibrium  is  easily  reduced  to  the  general  princi- 
ples of  Mechanics,  in  consequence  of  certain  circum- 
stances in  their  structure,  and  in  the  position  where 
they  are  placed.  It  is  necessary,  in  investigating  the 
conditions  of  equilibrium,  to  have  three  planes  per- 
pendicular to  each  other,  given  in  space,  as  one  of 
the  conditions  of  the  problem.  Three  such  planes 
are  determined  in  every  vessel : 

(1.)  All  vessels  are  so  built  that  their  opposite 
sides  are  symmetric,  and  in  loading  great  care  is  taken 
to  distribute  the  weight  equally.  The  two  symmetric 
parts  are  divided  from  each  other  by  an  imaginary 
plane  passing  through  the  middle  of  the  keel.  This 
is  called  the  diametrical  section,  or  plane  of  the  keel. 

(2.)  The  vessel  floats  at  the  surface  of  the  fluid, 
having  a  part  of  its  hull  immersed.  This  immer- 
sed part  is  equal  in  volume  to  a  mass  of  the  fluid 
whose  weight  is  equal  to  that  of  the  vessel ;  and  this 
part  may  be  conceived  to  be  separated  from  that  not 
immersed,  by  a  plane  formed  by  supposing  the  surface 
of  the  fluid  to  be  produced  through  the  body  of  the 
vessel.  This  plane  is  horizontal,  and  perpendicular 
to  the  plane  of  the  keel. 

The  part  immersed  is  called  the  hollow  of  the  ves- 
sel,  the  plane  which  separates  it  that  of  flotation. 

(3.)  The  point  of  application  of  the  resultant  of 
the  fluid  pressure  which  acts  upon  the  outer  surface 


PRACTICAL  MECHANICS.  223 

of  the  hollow,  is  situated  in  the  centre  of  gravity  of 
the  displaced  fluid,  or  in  that  of  the  hollow,  consid- 
ered as  homogeneous.  This  force  is  in  equilibrio 
with  the  weight,  and  the  centre  of  the  hollow  must 
therefore  He  in  the  same  vertical  line  with  the  centre 
of  gravity  of  the  vessel.  Both  these  points  are  sit- 
uated in  the  plane  of  the  keel,  and  a  plane  passed 
through  them  perpendicular  to  the  keel  will  be  ver- 
tical, and  also  perpendicular  to  the  plane  of  flotation. 

The  plane  thus  passed  through  the  two  centres 
corresponds  nearly  with  the  greatest  transverse  area 
of  the  vessel,  which  is  called  the  midship  section, 
and  we  may,  without  error,  consider  them  as  being 
one  and  the  same, 

235.  In  consequence  of  the  usual  figure  of  the  hol- 
low, which  is  much  fuller  near  the  midship  section 
than  at  its  extremities,  it  happens,  that,  although  the 
whole  weight  of  the  vessel  is  in  exact  equilibrium 
with  the  buoyant  forces,  the  partial  weights  and  par- 
tial buoyant  forces  which  act  upon  any  given  por- 
tion of  the  surface  of  the  hollow  are  not  in  equilibrio. 
If,  therefore,  as  is  the  fact,  the  materials  of  which  a 
vessel  is  constructed  are  neither  rigid  nor  perfectly 
well  fastened  together,  there  must  be  a  tendency  to 
bend  under  the  varying  action  of  the  two  forces. 

The  manner  in  which  this  occurs  may  be  under- 
stood from  the  following  explanation.  Fig.  76  rep- 
resents a  section  in  the  plane  of  the  keel  of  a  vessel 
of  the  usual  figure  ;  g  is  the  centre  of  gravity,  h  the 
centre  of  the  hollow  ;  if  each  of  these  forces  be 
considered  as  made  up  of  two  parts,  divided  in  their 
action  from  each  other  by  the  midship  section  m  s, 
the  centres  of  action  of  the  weight  will  be  at  points 
such  as  g'  and  g",  while  the  centres  of  the  buoyant 
forces  will  be  at  points  such  as  K  and  h",  nearer  to 
the  midship  section  than  g  and  g".     It  will  there- 


224 


PRACTICAL    MECHANICS. 


Fig.  76.  foj-e  be  obvious,  that  the  weight 

acts  at  a  mechanical  advantage 
upon  the  arm  of  a  lever  longer 
I  than  that  on  which  the  buoyant 
.force    does.      The    tendency  to 
bend  the  vessel  growing  out  of 
this  cause,  were  we  to   resolve 
the  weight  and   buoyant  forces 
into  a  greater  number  of  partial 
forces,  would  be  still  more  obvi- 
ous.    For  it  would  then  be  ap.. 
parent  that,  at  the  midship  sec- 
I  tion  and  its  neighbourhood,  the 
buoyant  force  is  in  excess,  while 
towards  the  stem  and  stern'  it  is 
more  and  more  in  defect.     All 
vessels,  therefore,  which  are  of 
jisuch  a  figure  as  to  possess  good 
I  properties  as  sailers,  which  we 
[shall  find  requires    them    to  be 
i  sharp  both  fore  and  aft,  have  a 
tendency  to  bend,  by  rising   in 
the  middle,  and  sinking  at  the 
"bs;  MM  IIIIIB^  This    tendency  produces 

I  in  all  large  vessels  a  change  of 
figure,  which  is  called  hogging. 

While  the  vessel  lies  upon  the 
{stocks,  and  the  keel  is  equally 
[supported   throughout,   the   ten- 
Idency   to    bend    does   not   take 
I  place  ;  but  the  forces  which  cause 
•it  are  called  into  action  at  the 
instant    of  launching,  when  the 
vessel  is  for  the  first  time  water 
borne.    Even  in  this  act,  and  be- 
fore any  additional  weight  in  the  form  of  equipment, 


^i':^! 


■i:^ 


PRACTICAL    MECHANICS.  225 

armament,  and  stores  has  been  taken  on  board,  the 
bend  in  the  deck  of  a  74  gun  ship  has  been  found  by 
observation  to  be  as  much  as  8  inches.  The  bend- 
ing force  acts  upon  the  vessel  during  its  whole  du- 
ration, and  is  the  most  efficient  cause  of  the  rapid 
rate  at  which  ships  of  war  cease  to  be  serviceable. 
It  is  at  any  rate  certain,  that  wood  which  will,  in  a 
good  mechanical  combination,  retain  a  sufficient  de- 
gree of  strength  for  ages  on  the  land,  soon  becomes 
so  weak  in  a  vessel,  even  without  rotting,  as  to  re- 
quire to  be  replaced,  or  the  vessel  to  be  condemned 
as  not  seaworthy. 

The  longer  the  vessel  and  the  more  acute  its  ex- 
tremities, the  greater  will  be  the  tendency  to  hog. 
This  tendency  may  be  partially  met  in  the  stowage 
of  vessels,  by  placing  the  greatest  weight  near  the 
midship  section,  and  leaving  the  parts  near  the  stem 
and  stern  free  from  lading.  But  this  method  is  rare- 
ly practicable  even  in  merchant  vessels,  and  in  ships 
of  war  is  out  of  the  question,  because  their  arma- 
ment, which  forms  a  large  portion  of  the  weight  they 
carry,  must  be  distributed  with  uniformity. 

236.  Vessels  are  usually  constructed  of  a  number 
of  frames  of  timber  at  right  angles  to  the  plane  of 
the  keel.  These^  are  bound  together,  and  the  hull 
rendered  water-tight  by  a  series  of  planks,  which 
cross  the  midship  section  at  right  angles,  and  the 
other  frames  at  angles  more  or  less  acute  as  they 
recede  from  that  section.  Farther  strength  is  at- 
tempted to  be  given  by  a  series  of  planks,  called  the 
ceiling,  which  lie  parallel  to  the  former,  on  the  in- 
side of  the  frames.  The  frames  and  planks,  in  cross- 
ing each  other,  therefore  formed  figures  differing  but 
little  from  a  parallelogram,  and  are  under  the  cir- 
cumstances of  a  gate  without  a  diagonal  brace,  or  a 
door  without  panels.     Both  of  these  structures,  as 


226  PRACTICAL   MECHANICS. 

is  well  known,  would  speedily  lose  their  original  fig- 
ure, and  finally  fall  to  pieces  under  the  action  of  their 
own  weight.  In  the  same  manner,  a  ship  tends  to 
bend,  and  is  finally  destroyed,  by  the  excess  of  the 
action  of  the  weight  over  that  of  the  buoyant  force, 
at  points  distant  from  the  midship  section  ;  and  this 
tendency  is  not  met,  as  in  the  gate,  by  diagonal  bra- 
cing, or  in  the  door,  by  filling  up  the  frame  with  pan- 
els. 

The  simplest  mode  of  obviating  this  defect  would, 
we  conceive,  have  been  that  long  adopted  in  the  gates 
of  canal  locks,  in  which  the  planking  is  often  placed 
in  a  diagonal  position.  In  imitation  of  this,  it  would 
have  been  sufficient  to  have  laid  the  ceiling  planks  in 
a  diagonal  position,  inclining  downward  in  both  di- 
rections, from  the  midship  section  towards  the  stem 
and  stern. 

Seppings,  a  British  naval  architect,  in  the  year 
1812,  proposed  another  method  founded  on  the  same 
principles.  He  suppressed  the  ceiling  plank  alto- 
gether, substituting  for  it  a  series  of  diagonal  timbers 
extending  each  way  downward  from  the  midship  sec- 
tion. Above  the  main-deck  he  introduced  shorter 
diagonal  braces,  and  even  the  planking  of  the  decks 
was  laid  diagonally. 

He,  in  addition,  proposed  to  introduce  the  principle 
of  the  panel,  by  filHng  every  vacant  space  with  pieces 
of  timber.  In  many  of  the  British  ships  also,  the 
frames,  which  have  usually  spaces  between  them, 
were  made  to  touch,  so  that,  by  calking,  they  might  be 
rendered  water-tight.  This,  however,  rather  adds  to 
the  inherent  defect  of  vessels.  It  did  not  occur  to 
Seppings  that,  in  the  works  of  nature  and  in  the  most 
skilful  productions  of  human  art,  strength  is  gained  in 
two  ways,  namely,  by  a  better  arrangement  of  their 
materials,  and  by  diminishing  their  weight  as  much 


PRACTICAL   MECHANICS.  227 

as  possible.  It  cannot  be  doubted  that  the  quantity 
of  timber  which  is  employed  in  the  structure  of  a 
vessel  is  far  more  than  is  sufficient  for  strength, 
were  it  skilfully  distributed  and  arranged.  Much 
therefore  remains  to  be  done  towards  the  perfection 
of  naval  architecture,  which,  so  far  as  resistance  to 
flexure  is  concerned,  had  remained  in  the  same  state 
from  its  earliest  origin  to  the  time  of  Seppings. 

The  success  of  Seppings's  method  is,  notwith- 
standing,  such  that,  in  a  seventy-four  gun  ship,  the 
bend  of  the  deck  which  takes  place  in  the  mere  act 
of  launching  was  reduced  from  8  inches  to  2  inches. 
Subsequent  experience  has  shown  the  greater  dura- 
bility of  vessels  built  on  his  construction. 

At  present,  diagonal  braces  of  iron  are  much  used 
instead  of  the  diagonal  timbers  of  Seppings. 

237.  The  equilibrium  of  a  vessel,  under  the  action 
of  the  weight  and  the  buoyant  force,  is  not  permanent, 
but  is  liable  to  be  disturbed  by  the  action  of  the  wind 
upon  its  sails,  or  by  the  oscillations  of  the  fluid  on 
which  it  floats.  Now  the  centre  of  the  hollow  lies 
in  all  cases  below  the  surface  of  the  water,  and  the 
centre  of  gravity  is  most  frequently  above  that  sur- 
face. The  point  of  support  being  thus  below  the 
point  at  which  the  weight  acts,  the  condition  would 
be  that  of  tottering  equilibrium,  were  it  not  for  the 
circumstance  we  shall  explain. 

In  the  vessel  whose  midship  section  is  represented 
Fig.  77,  the  respective  centres  of  gravity  and  of  the 
hollow  are  at  the  points  g  and  li.  Let  us  suppose 
the  vessel  to  be  inclined  in  such  manner  that  the  level 
of  the  surface  of  the  fluid  no  longer  corresponds  with 
the  line  a  b,  but  with  c  d.  It  will  then  be  obvious, 
that  while  the  line  of  direction  of  the  centre  of  grav- 
ity has  changed  in  its  relative  position  in  the  vessel 
from  g  e  to  g  f,  the  centre  of  the  hollow  will  have 


228  PRACTICAL    MECHANICS. 

Fig.  77. 


moved  outward  towards  the  side  h  d.  If  it  have 
changed  its  position  to  a  point  such  as  li",  which  is 
more  distant  from  the  plane  of  the  keel  than  the  line 
of  direction  gf,  the  buoyant  force  will  act  at  a  me- 
chanical advantage  over  the  force  of  gravity,  and  the 
vessel  will  tend  to  return  to  her  original  position. 
But  if  the  point  li  fall  in  the  line  gf,  then  the  two 
forces  will  still  counterbalance  each  other,  and  there 
will  be  no  tendency  to  return.  If,  again,  it  should 
fall  between  g  f  and  g  e,  the  weight  will  act  to  a 
mechanical  advantage,  and  the  condition  will  be  that 
of  tottering  equilibrium,  under  which  the  vessel  could 
be  upset.  It  will  easily  be  seen,  that  by  a  proper 
construction  of  vessel  and  distribution  of  the  cargo, 
the  first  of  these  conditions  may  be  attained,  namely, 
that  when  the  vessel  is  caused  to  incline  by  the  ac- 
tion of  an  extrinsic  force,  the  buoyant  pressure  of  tho 
water  may  for  a  time  be  made  to  act  more  forcibly, 
and  thus  the  vessel  be  caused  to  return  to  her  primi- 
tive position  ;  here  the  two  forces  are  again  in  equi- 
librium,  and  thus  the  vessel  will  continue  to  perform  a 


PRACTICAL   MECHANICS.  229 

series  of  oscillations  from  side  to  side,  an  act  which  is 
called  rolling.  When  the  disturbing  force  is  that 
of  the  wind  acting  upon  sails,  the  return  towards  the 
original  position  is  aided  by  the  fact  that  the  wind 
acts  less  directly  on  the  sails  when  the  vessel  is  in- 
clined, and  its  centre  of  action  is  nearer  the  horizon- 
tal surface,  in  consequence  of  which  its  intensity  is 
diminished,  as  upon  a  lever  of  a  shorter  arm. 

It  may,  however,  happen,  that  in  any  vessel  what- 
soever, the  effect  of  the  wind  on  the  sails  may  be 
such,  that  if  they  do  not  themselves  give  way,  the 
vessel  will  be  carried  beyond  the  point  at  which  the 
relative  motions  of  the  centre  of  the  hollow  and  of 
the  line  of  direction  of  the  centre  of  gravity  will  ful- 
fil the  required  condition.  From  the  position,  al- 
most vertical,  in  which  the  timbers  on  which  the 
decks  lie  are  now  placed,  the  vessel  is  said  to  be 
thrown  on  her  beam-ends,  and  has  no  longer  any 
tendency  to  return  to  an  upright  position.  There  is, 
however,  a  remedy  which  is  within  reach.  The 
condition  of  stability  will  be  resumed  by  lowering  the 
position  of  the  centre  of  gravity,  and  this  may  be  done 
by  getting  rid  of  the  weight  of  the  masts,  sails,  and 
rigging.  For  this  purpose  it  is  not  necessary  to  cut 
away  the  masts.  These  are  so  large  that  their  re- 
spective strength  is  not  sufficient  to  resist  the  action 
of  their  own  weight.  They  would  therefore  break 
when  lying  in  a  horizontal  position,  were  they  not 
strengthened  by  the  cordage  known  by  the  name  of 
lanyards.  It  is  sufficient  to  cut  these  on  the  upper 
side  of  the  mast,  and  the  latter,  being  no  longer  sup- 
ported, will  give  way. 

238.  In  the  act  of  rolling,  the  centre  of  the  hollow 
revolves  around  a  point  which  is  called  the  metacen- 
tre,  and  the  greater  or  less  stability  of  the  vessel  will 
depend  upon  the  position  of  this  point.     The  higher 


230  PRACTICAL   MECHANICS. 

this  metacentre  is  situated,  the  greater  the  stability4 
The  metacentre  may  be  raised,  and  the  stability  en- 
hanced, by  increasing  the  breadth  of  beam,  and  by 
making  the  extreme  breadth  of  the  vessel  lie  higher 
than  the  level  of  the  surface  of  the  water.  .  The  sta- 
bility may  also  be  increased  by  lowering  the  position 
of  the  centre  of  gravity,  when  this  can  be  done  with- 
out increasing  the  weight  with  which  the  vessel  is 
loaded  beyond  proper  limits.  When,  however,  a  ves- 
sel is  so  deeply  laden  that  the  draught  of  water  ex- 
ceeds half  the  breadth  of  beam,  or  when  the  extreme 
breadth  is  immersed,  the  advantage  derived  from  this 
source  ceases. 

Analysts  have  shown  that  the  stability  will  be 
greatest  when  the  curves  drawn  on  the  bottom  of  the 
vessel  are  circles  whose  centre  lies  in  the  point  of 
application  of  the  disturbing  forces.  This  figure 
cannot  be  given  when  the  disturbing  force  is  the  wind 
applied  to  sails,  but  may  be  used  when  the  oscilla- 
tions are  principally  due  to  the  waves.  Figures  of 
this  description  are  to  be  found  in  portions  of  the 
rind  of  a  cocoanut  or  of  the  skin  of  an  orange ;  and 
these,  when  thrown  upon  the  water,  will  float  with 
their  pointed  ends  upward,  whatever  be  the  vioJence 
of  the  winds  and  waves.  Upon  this  principle  Mr. 
Greathead  constructed  his  life-boat,  of  which  Fig.  78 
is  a  draught. 

This  vessel,  it  has  been  found,  cannot  be  upset. 
Nor  can  it  sink,  because  all  the  space  beneath  the 
seats,  and  at  the  bow  and  stern,  are  filled  in  with 
cork,  which  has  so  little  density  that  the  life-boat, 
when  loaded  with  as  many  persons  as  it  can  carry, 
and  when  all  the  vacant  space  is  filled  with  water,  is 
still  buoyant.  Airvessels  might  be  substituted  for 
cork,  but  are  liable  to  injury  ;  and  a  patent  has  re- 
cently been  taken  out  for  using  vessels  filled  with  by- 


PRACTICAL   MECHANICS.  231 

Fig.  78. 


drogen  gas,  in  order  to  render  a  life-boat  buoyant. 
This  method,  however  plausible,  is  of  little  or  no 
value,  for  the  difference  between  the  weights  of  these 
bulks  of  common  air  and  hydrogen,  however  differ- 
ent their  densities,  does  not  bear  a  sufficient  propor- 
tion to  the  whole  weight  of  the  vessel,  to  make  any 
important  difference  in  its  buoyancy. 

239.  When  a  vessel  has  masts  and  sails,  it  is  not 
advantageous  to  increase  the  stability  beyond  a  cer- 


232  PRACTICAL    MECHANICS. 

tain  limit,  because  in  this  case  the  motion  of  rolHng 
will  be  performed  with  a  degree  of  violence  which 
may  carry  away  the  masts,  or  may  even  cause  the 
vessel  to  founder.  With  an  equal  degree  of  stability, 
the  act  of  rolling  may  be  performed  in  different  times, 
and,  the  more  slowly  it  is  performed,  the  less  will  be 
the  danger  with  which  it  is  attended.  The  centre  of 
gravity  and  the  quantity  of  the  vessel's  lading  re- 
maining unchanged,  the  velocity  of  rolling  may  be 
lessened  by  stowing  the  weight  as  far  as  possible  from 
the  plane  of  the  keel.  A  knowledge  of  this  fact  has 
led  to  some  important  improvements  in  seamanship 
and  in  naval  architecture.  Thus,  in  ships  of  war,  it 
is  no  longer  customary  to  house  the  guns,  as  it  is 
styled,  or  draw  them  in  board  for  the  purpose  of 
shutting  the  ports ;  but  the  ports  have  holes  in  them, 
and  are  fitted  with  tarpaulin  cases,  in  which  the  muz- 
zles of  the  guns  are  enclosed.  So  also  the  transverse 
section  of  large  vessels  has  been  in  some  cases 
changed,  and  always  with  advantage.  These  sec- 
tions formerly,  after  rising  to  their  extreme  breadth 
a  short  distance  above  the  load-water-line,  fell  rapid- 
ly inward,  forming  a  concave  curve.  In  the  U.  S. 
ship  Ohio,  for  the  first  time,  this  method  was  de- 
parted from ;  and,  although  the  width  of  the  spar- 
deck  is  less  than  the  extreme  breadth,  the  diminution 
is  effected  by  means  of  a  convex  curve.- 

The  dangers  arising  from  too  great  a  degree  of 
stability  were  well  illustrated  in  the  case  of  two 
Spanish  ships  of  war.  That  nation,  by  the  applica- 
tion of  scientific  principles  to  naval  architecture,  had 
reached  a  degree  of  perfection  which  the  French  and 
English  despaired  of.  As  an  instance  of  this,  we 
may  cite  the  fact,  that  when  the  French  had  almost 
abandoned  the  attempt  to  build  three-decked  ships, 
and  those  constructed  by  the  English  were  so  infe- 


PRACTICAL    MECHANICS.  233 

rior  in  their  qualities  as  sea-boats,  that  the  model  of 
one  (the  Victory)  which  had  been  approved  in  ser- 
vice was  preserved  by  continual  rebuilding,  the 
Spaniards  were  successful  in  building  a  vessel  of 
four  decks.  Among  the  vessels  built  at  this  period 
in  Spain  were  two  of  two  decks.  These  never  left 
the  port  without  being  compelled  to  return  dismasted 
by  the  first  gale  of  wind.  This  defect  was  remedied 
by  converting  them  into  vessels  of  three  decks,  by 
which  their  excess  of  stability  was  diminished,  and, 
with  it,  the  violence  of  the  rolling  motion. 

In  our  steamboats,  the  engines  and  boilers  are 
often  placed  on  the  wheel-guards,  while  the  English 
have  taken  great  pains  to  place  these  heavy  weights 
as  low  and  as  near  as  possible  to  the  plane  of  the 
keel.  It  may  be  doubted  whether  the  practice  of  the 
Americans  in  this  respect  is  not  superior,  even  in 
sea-going  vessels,  inasmuch  as  the  rapidity  of  the 
rolling  motion  is  increased  in  steam-vessels  by  the 
lessening  of  the  length  of  the  masts  and  of  the  quan- 
tity of  sail,  and  therefore  requires  to  be  counteracted 
by  raising  the  weight  carried  in  the  hull  of  the  ves- 
sel. At  all  events,  it  must  be  admitted,  that  if  the 
American  practice  is  erroneous  in  diminishing  the 
stability,  the  English  method  is  not  less  so  in  increas- 
ing the  violence  of  the  rolling  motion,  and  the  most 
advantageous  position  of  these  weights  will  probably 
be  found  between  these  two  extremes. 

240.  Vessels  are  also  liable  to  an  oscillating  mo- 
tion in  a  direction  from  stem  to  stern,  which  is  called 
pitching.  By  this  the  vessel  may  be  rendered  wet,  the 
masts  and  rigging  may  be  strained  or  carried  away, 
and  there  are  even  cases  in  which  it  has  caused  the 
vessel  to  founder.  In  consequence  of  the  great  pro- 
portion which  the  length  bears  to  the  breadth  of  the 
vessel,  these  oscillations  are  more  influenced  by  the 


234  PRACTICAL    MECHANICS. 

waves  than  by  the  winds.  While  danger  arises  in 
the  act  of  rolling  from  too  great  a  frequency  in  ihe 
oscillations,  the  danger  of  pitching  is  increased  by 
the  extent  of  the  arc  in  which  the  motion  is  perform- 
ed, and  lessened  by  increasing  the  number  of  oscil- 
lations in  a  given  time.  In  order  to  ^nitigate  the 
violence  of  pitching,  the  weight  in  stowage  ought 
therefore  to  be  placed  as  near  as  possible  to  the  mid- 
ship section.  In  the  building  and  rigging,  all  heavy 
masses  near  the  bow  and  stern  are  therefore  to  be 
avoided.  In  conformity  with  this  rule,  the  heavy 
figure-heads,  quarter-galleries,  and  poops, with  which 
ships  were  formerly  encumbered  rather  than  embel- 
lished, are  now  omitted. 

241.  So  long  as  a  vessel  remains  at  rest,  the  pres- 
sure of  the  fluid  acts  upon  it  equally  in  all  directions  ; 
but,  as  soon  as  it  is  set  in  motion  from  any  cause,  a 
resistance  is  opposed  to  its  progress.  In  a  given 
vessel,  this  resistance  appears  at  first  to  follow  the 
law  which  is  usually  stated  as  that  of  fluid  resistance, 
namely,  that  of  the  square  of  the  velocity.  As  the 
velocity  increases,  a  new  cause  of  resistance  appears. 
This  is  owing  to  the  fact  that  a  wave  is  usually  raised 
in  front  of  the  vessel,  while  the  fluid  does  not  close 
behind  it,  and  thus  the  propelling  force  must  be  in 
part  exerted  to  raise  the  vessel  up  an  inclined  plane. 
This  cause  of  resistance  is  stated  by  Juan  to  follow  the 
law  of  the  fourth  power  of  the  velocity.  By  more 
recent  investigations,  it  seems  to  have  been  proved, 
that  when  the  wave  moves  in  the  fluid  at  the  same 
rate  with  the  vessel,  this  rather  aids  than  opposes  the 
motion.  The  velocity  of  this  wave  depends  upon  the 
magnitude  and  figure  of  the  channel ;  we  cannot,  there- 
fore, state  any  general  rule  by  which  it  is  governed. 

The  resistance  to  the  progressive  motion  of  a  ves- 
sel is  greatest  when  the  prow  is  a  vertical  plane. 


PRACTICAL   MECHANICS. 


235 


Fig.  79. 


When  a  wedge  is  applied  to  such  a  surface,  the  re- 
sistance is,  according  to  theory,  diminished  in  the 
ratio  of  the  square  of  the  cosine.  Hence  the  more 
acute  the  prow,  the  less  will  be  the  resistance.  It 
has  been  attempted  to  investigate  analytically  the 
figure  of  the  prow  of  least  resistance,  but  as  the  law 
we  have  stated  is  not  absolutely  true,  the  investiga- 
tion is  of  no  practical  value.  The  void  space  left 
behind  the  vessel  is  diminished 
by  giving  to  the  stern  the  shape 
of  a  wedge  also,  and  the  velo- 
city given  by  a  constant  force 
will  be  greatest  when  the  wedge 
of  the  stern  is  more  acute  than 
that  of  the  bow. 

A  rectangle  terminated  by 
wedges  is  far  from  being  the 
best  form  for  the  horizontal  sec- 
tion of  a  vessel.  The  resistance 
may  be  lessened  by  giving  to  this 
section  the  figure  of  a  continu- 
ous curve,  from  stem  to  stern. 
At  the  load-water-line,  it  would 
appear  from  experiment,  the 
main  breadth  should  be  at  a 
distance  of  three  eighths  from 
the  bow,  and  five  eighths  of  the 
length  from  the  stern  ;  and  the 
curves  in  this  plane  may  be  con- 
vex in  both  directions.  Below 
the  surface  of  the  water  the  curves 
should  continue  to  be  convex  to- 
wards the  bow,  but  should  grad- 
ually become  concave  towards 
the  stern.  Water-Hnes  of  this 
character  are  represented 
Fig.  79. 


/ 


m 


236  PRACTICAL   MECHANICS. 

In  the  vessels  recently  built  in  the  port  of  New- 
York,  models  founded  on  this  principle  have  been 
adopted.  The  floor  is  nearly  level  from  stem  to 
stern,  and  the  midship  section  is  as  nearly  rectangu- 
lar as  is  consistent  with  the  proper  connexion  of  the 
timbers  which  compose  the  frame.  These  vessels, 
therefore,  draw  much  less  water  than  those  of  the 
old  construction,  but,  to  the  surprise  of  nautical  men, 
they  have  also  been  found  better  sailers. 

242.  Vessels  may  be  propelled  by  the  wind  act- 
ing upon  sails  ;  by  the  force  of  men  acting  upon  oars 
or  paddles  ;  or  by  the  steam-engine,  acting  usually 
upon  a  wheel  resembling  the  undershot  water-wheel. 

243.  When  the  wind  acts  upon  sails,  it  is  thrown 
off  in  consequence  of  its  elasticity,  and  the  direction 
of  the  resulting  force  is  not  that  of  the  wind,  but  at 
an  angle  formed  by  a  line  bisecting  the  directions  of 
the  direct  and  reflected  current  of  air.  Were  the 
surface  of  the  sail  a  plane,  and  were  the  reflected 
current  not  affected  by  the  succeeding  portions  of 
the  direct  current,  this  line  would  be  perpendicular 
to  the  plane  of  the  sail.  It  is  impossible  to  take  into 
account  the  deviation  which  is  due  to  the  action  of 
the  direct  upon  the  reflected  current,  and  we  shall 
therefore  assume  that  the  direction  in  which  the  wind 
tends  to  impel  a  sail  is  perpendicular  to  its  surface. 
In  this  direction,  the  vessel,  if  its  water-lines  were 
circles,  would  be  caused  to  move,  provided  it  could 
be  maintained  in  that  course.  But  the  force  of  the 
wind  also  tends  to  make  the  sail  revolve,  until  it  come 
into  a  position  perpendicular  to  the  direction  of  the 
wind.  It  will  therefore  require  some  apparatus  to 
prevent  the  vessel  from  obeying  this  tendency  to 
revolve.  The  horizontal  section  of  vessels  at  the 
level  of  the  water  is  far  from  being  circular.     The 


PRACTICAL    MECHANICS.  237 

proportion  of  the  length  to  the  breadth  is  rarely 
less  than  3  ;  1,  and  usually  greater.  The  bow  forms 
an  acute  wedge,  the  direct  progress  of  which  is 
but  Uttle  resisted,  while  the  resistance  to  a  lateral 
motion  is  little  less  than  would  be  sustained  by  a 
plane  surface.  It  happens,  therefore,  from  the  ten- 
dency of  the  sail  to  move  perpendicular  to  its  own 
surface,  and  the  great  excess  of  the  resistance  to  a 
lateral  motion,  that  even  when  the  wind  blows  per- 
pendicular to  the  plane  of  the  keel,  the  course  will 
not  deviate  much  from  the  direction  of  that  plane, 
which  is  hence  assumed  to  be  the  actual  course  of 
the  vessel,  and  any  deviation  from  it  is  applied  as  a 
correction  under  the  name  of  leeway.  The  lee- 
way decreases  with  an  increase  in  the  length  of  the 
vessel  and  of  the  draught  of  water  ;  it  is  also  less  in 
sharp  than  in  full-built  vessels.  It  is  greatest  when 
the  angle  the  direction  of  the  wind  makes  at  the  bow 
with  the  plane  of  the  keel  is  least,  and  becomes  0 
when  the  wind  is  directly  aft. 

244.  Vessels  may  be  divided  into  two  classes.  In 
the  first,  the  most  important  sails  are  attached  to 
yards,  whose  primitive  position  is  at  right  angles  to 
the  masts  and  to  the  plane  of  the  keel.  Such  ves- 
sels are,  in  consequence,  said  to  be  square  rigged.  In 
the  other  class,  the  primitive  position  of  the  principal 
sails  is  in  the  plane  of  the  keel.  In  the  first  class, 
the  yards  may  be  braced  round  until  the  angle  they 
make  with  the  plane  of  the  keel  is  diminished  to 
45°,  or  even  to  40°.  In  the  second  class,  the  sails 
will  not  receive  the  wind  in  such  a  manner  as  to 
be  impelled  by  it  until  they  have  deviated  from  the 
plane  of  the  keel  to  an  angle  of  more  than  20°.  In 
both  cases,  the  angle  which  the  course  makes  with 
the  plane  of  the  keel,  without  allowance  for  lee- 
way, may  be  considerably  less  than  90°,  and  may 
19 


238 


PRACTICAL    MECHANICS. 


be  taken  in  many  fore-and-aft  rigged  vessels  as  even 
less  than  45°.  Whenever  this  angle  is  less  thaa 
90°,  the  vessel,  in  its  oblique  course,  approaches 

Fig.  80. 


PRACTICAL    MECHANICS.  239 

towards  the  point  whence  the  wind  appears  to  blow, 
and  is  said  to  beat  or  ply  to  windward.  It  may  thus 
happen  that,  by  pursuing  two  obUque  courses  in  suc- 
cession, a  vessel  may  reach  a  point  whence  the  wind 
appeared  to  blow  when  it  set  out. 

In  Fig.  80,  the  wind  is  represented  as  blowing 
from  the  north.  The  lower  of  the  two  fore-and-aft 
rigged  vessels  may  therefore  pursue  a  course  due 
northwest,  and,  on  coming  to  the  point  marked  N  W, 
may  tack,  and  thence  pursue  the  course  on  which 
the  upper  vessel  is  represented  as  proceeding,  to  the 
northeast. 

In  plying  to  windward,  a  part  of  the  velocity  of 
the  vessel,  if  resolved  into  two  components,  is  in  di- 
rect  opposition  to  the  velocity  of  the  wind,  and  the 
velocity  with  which  the  wind  strikes  the  sails  is  the 
8um  of  the  two.  Hence  the  action  of  the  wind  in 
oblique  courses  is  in  ased,  and  the  velocity  of  the 
vessel  with  it.  Wii  the  last  velocity  the  resistan- 
ces increase  also,  ana  in  a  higher  ratio ;  hence  there 
is  a  limit  to  the  increase ;  but  the  singular  results 
are  thus  reached,  that  a  vessel  of  good  construction 
shall  ply  to  windward  on  two  courses  to  a  given 
point,  in  less  time  than  it  can  return  before  the  wind 
and  that  the  velocity  of  the  vessel  in  beating  may  be 
greater  than  that  of  the  wind  itself. 

Both  of  these  facts  were  observed  by  Juan  on  the 
ferry  between  Cadiz  and  Matagorda. 

When  before  the  wind,  on  the  other  hand,  the  ve- 
locity  through  the  water  can  never  equal  that  of  the 
wind ;  but,  as  the  course  becomes  more  and  more 
oblique,  the  velocity  of  a  well-moulded  vessel  will 
increase  up  to  a  certain  point,  when  it  will  again 
diminish.  The  position  of  maximum  velocity  in  a 
square-rigged  vessel  of  good  figure,  is  usually  that 
in  which  the  direction  of  the  wind  is  at  right  angles 
to  the  plane  of  the  keel. 


240  PRACTICAL    MECHANICS. 

The  method  of  plying  to  windward  was,  in  all 
probability,  unknown  to  the  ancients,  and  was,  as  is 
believed,  introduced  in  Europe  by  the  navigators  of 
Amalfi,  in  Italy.  To  this  day  it  does  not  seem  to 
be  known  to  the  people  of  China  and  Japan.  It  is  a 
remarkable  fact,  that  while  these  comparatively  civ. 
ilized  nations  are  deficient  in  this  point  of  seaman- 
ship, a  rude  people,  the  natives  of  the  Ladrone  Isl- 
ands, in  their  neighbourhood,  were  found  in  posses- 
sion of  the  most  perfect  vessel  which  has  ever  been 
contrived  for  this  purpose,  and  which,  from  its  prop- 
erties of  sailing,  has  been  called  the  flying  proa. 

The  body  of  a  flying  proa  resembles  that  of  a  ves- 
sel which  has  been  cut  in  two  along  the  plane  of  the 
keel,  and  having  this  plane  planked  up.  It  thus  pre- 
sents a  surface  which  is  only  resisted  in  its  progress, 
ive  motion  half  as  much  as  that  of  a  vessel  of  the 
usual  form,  and  having  an  equal  midship  section.  As 
it  could  not  remain  upright  with  a  figure  of  this  kind, 
there  is  a  long  outrigger  formed  of  spars,  which  pro- 
jects from  the  curved  side,  and  is  terminated  in  a 
solid  piece  of  wood  fashioned  into  the  form  of  a  vessel. 
When  the  vessel  tends  to  incline  in  this  direction,  the 
buoyancy  of  the  outrigger  opposes  the  inclination ; 
and,  should  the  wind  tend  to  incline  it  in  the  opposite 
direction,  the  weight  of  the  outrigger  produces  a 
similar  eflect.  The  proa,  therefore,  floats  with  lit- 
tle variation  from  an  erect  position.  The  plane  sur- 
face is,  on  an  oblique  course,  turned  towards  the 
wind,  and,  in  connexion  with  the  outrigger,  opposes 
such  a  resistance  to  leeway  as  to  render  it  almost 
insensible.  The  sails  are  triangular  in  figure,  and 
will  draw  at  a  less  angle  with  the  wind  than  those  of 
any  other  vessel.  This  advantage  is  partly  gained 
by  an  arrangement  similar  to  that  of  the  sails  of  the 
Chinese.     This  consists  in  a  number  of  slender  rod^ 


PRACTICAL    MECHANICS.  241 

Stretched  along  the  sail  in  a  horizontal  direction, 
by  which  the  sail  is  prevented  from  swelling  or  bel- 
lying except  in  the  vertical  direction. 

In  connexion  with  this  last  circumstance,  it  may 
be  stated  that  the  sails  of  European  vessels  were  for- 
merly 30  cut  as  to  permit  them  to  bag  or  belly  con- 
siderably. A  better  practice  is  now  in  use  in  this 
country,  where  they  are  cut  to  lie  as  flat  as  possible  ; 
and,  in  order  to  ensure  this,  in  fore-and-aft  rigged 
vessels,  the  sail  is  not  merely  fixed  at  its  two  lower 
corners  as  formerly,  but  is  tied  down  to  the  boom  at 
short  intervals.       ^ 

The  periauga  used  in  the  waters  of  New- York 
and  New-Jersey  has  also  excellent  qualities  in  ob- 
lique courses.  Its  name  would  seem  to  import  that  it 
is  in  part  borrowed  from  the  pirogue  of  the  Indians. 
But  the  great  tendency  which  its  canoe  shape  and 
small  draught  of  water  would  give  it  to  leeway, 
is  counteracted  by  lee-boards,  which  are  borrowed 
from  the  Dutch  schuyt.  For  these  a  sliding  keel  or 
centre-board  is  now  usually  substituted,  and  the 
shape  of  the  hull  approaches  more  nearly  to  that  of 
other  vessels. 

The  pilot-boat  schooner  of  the  United  States,  how- 
ever, appears  to  combine  a  greater  number  of  good 
qualities,  of  which  its  capacity  of  lying  near  the  wind 
is  but  one,  than  any  other  vessel. 

245.  The  direction  of  a  vessel  on  an  oblique 
course  is  principally  maintained  by  the  trim  of  the 
sr*ils.  When  these  are  so  distributed  that  the  sever- 
al actions  of  the  wind  upon  them  are  in  equilibrio 
around  the  vertical  axis  of  the  vessel,  there  is  no  ten. 
dency  to  a  change  of  course.  Such  a  trim  is,  how- 
ever, never  practised,  because  it  is  possible  that,  in 
oblique  courses,  the  action  of  the  wind  may  become 
sufficient  to  upset  the  vessel.     When  a  vessel  is  rig- 


242  PRACTICAL    MECHANICS. 

ged  with  square  sails,  security  from  this  danger  is 
best  attained  by  turning  the  head  of  the  vessel  from 
the  direction  of  the  wind.  In  this  case  the  vessel  is 
said  to  bear  away,  and  the  sails  ought  to  be  so  trim- 
med that  there  shall  be  a  constant  tendency  to  this 
change  of  course.  In  fore-and-aft  rigged  vessels, 
security  from  this  danger  is  best  attained  by  turning 
the  head  towards  the  wind,  until  the  sails  shake  r  they 
are  then  said  to  lujf. 

The  tendency  to  bear  away  or  to  luff  in  these  two 
cases  requires  to  be  counteracted.  This  is  done  by 
the  action  of  the  rudder.  The  rudder  is  a  flat  body 
of  wood  hung  upon  the  stern-post.  When  a  vessel 
is  in  motion,  a  current  of  equal  velocity  sets  along 
both  sides.  If,  now,  the  rudder  be  caused  to  change 
its  primitive  position  in  the  plane  of  the  keel,  it  in- 
terrupts the  flow  of  the  current  on  the  side  towards 
which  it  is  incHned,  and  by  this  interruption  causes 
the  vessel  to  turn  on  its  vertical  axis  towards  the  side 
on  which  the  rudder  is  protruded.  The  rudder  also 
serves  to  change  the  direction  of  the  course  of  ves- 
sels. It  would  appear,  from  an  analytic  investiga- 
tion,  that  if  the  surface  of  the  vessel  were  a  plane, 
the  rudder  would  produce  the  greatest  efl^ect  when 
it  makes  with  that  surface  an  angle  54°  44'.  But, 
in  consequence  of  the  wedge-like  shape  of  the  stern, 
the  most  advantageous  angle  with  the  plane  of  the 
keel  is  found  in  practice  to  lie  between  45°  and  48°. 

The  figure  of  the  water-lines  towards  the  stern, 
which  we  have  stated  ought  to  be  concave  curves,  is 
also  advantageous  to  the  action  of  the  rudder.  Ves- 
sels having  such  curves  are  said  to  be  clean  aft,  and 
they  always  steer  easily. 

It  will  be  obvious,  from  what  has  been  said  of  the 
manner  in  which  the  rudder  acts,  that  it  has  no  ef- 
fect when  the  vessel  is  not  moving  through  the  water. 


PRACTICAL    MECHANICS.  243 

246.  Vessels  in  plying  to  windward  may  change 
their  course  in  two  different  ways,  called  tacking,  and 
veering  or  wearing.  In  tacking,  the  head  of  the 
vessel  is  thrown  towards  the  wind  by  the  action  of 
the  rudder,  until  the  direction  of  the  plane  of  the  keel 
is  the  same  as  that  of  the  wind.  The  sails  shake  in 
this  position,  in  fore-and-aft  rigged  vessels,  and  are 
taken  aback,  in  square-rigged  vessels.  By  these 
means  the  velocity  of  the  vessel  is  lessened,  and, 
should  it  be  altogether  destroyed,  the  manoeuvre  can- 
not be  performed.  But  if  sufficient  velocity  is  left 
to  cause  the  vessel  to  obey  the  rudder,  the  action  ot 
the  wind  on  the  sails  is  caused  to  aid  in  the  comple- 
tion of  the  manoeuvre.  This  is  done  by  keeping  the 
head-sails  in  the  position  in  which  they  were  before 
the  tack  commenced,  while  the  sails  abaft  are  either 
permitted  to  obey  the  impulse  of  the  wind,  or  are 
trim.med,  by  bracing  the  yards  around,  to  receive  it 
from  the  opposite  side.  Finally,  the  head-sails  are 
either  permitted  to  obey  the  impulse  of  the  wind,  or 
braced  round. 

After  the  tack  is  performed,  the  vessel  will  be 
some  time  in  regaining  its  previous  velocity.  Du- 
ring this  time,  the  course  of  the  vessel  by  the  com- 
pass, or  as  observed  from  the  shore,  will  be  continu- 
ally changing,  although  the  apparent  angle  of  the 
wind  with  the  plane  of  the  keel,  as  observed  in  the 
vessel,  will  remain  unvaried.  This  is  owing  to  the 
fact  that  the  apparent  direction  of  the  wind  is  that  of 
the  resultant  of  its  own  velocity  and  that  of  the  ves- 
sel. This  circumstance  is  peculiarly  worthy  of  no- 
tice, inasmuch  as  the  observation  of  it  led  Bradley  to 
his  brilliant  discovery  of  the  cause  of  the  aberration 
of  the  fixed  stars,  and  thus  to  a  direct  proof  of  the 
revolution  of  the  earth  around  the  sun  in  an  annual 
orbit. 


244  PRACTICAL    MECHANICS. 

Vessels  rigged  fore-and-aft  are  more  certain  to 
perform  the  operation  of  tacking  than  those  which  are 
square-rigged  ;  fast-sailing  and  sharp-built  vessels  do 
it  better  than  those  which  are  full-built.  The  opera- 
tion is  impeded  by  waves,  and  hence  ships  seldom  re- 
sort to  it  at  sea,  and  more  particularly  in  naval  ac- 
tions, because  a  failure  to  complete  it,  or,  as  it  is  called, 
missing  stays,  leaves  the  vessel  for  a  time  helpless. 

247.  The  manoeuvre  of  veering  is  performed  by 
turning  the  head  of  the  vessel  from  the  wind,  trim- 
ming the  sails  to  correspond  with  the  revolution  until 
the  plane  of  the  keel  makes  the  same  angle  with  the 
wind  as  before,  but  on  the  opposite  side.  In  the  act 
of  tacking,  the  sails  shake  or  are  taken  aback ;  in 
the  act  of  veering,  the  sails  are  kept  full,  and  the  ves- 
sel is  for  an  instant  exactly  before  the  wind.  The 
act  of  veering  is  performed  with  greater  facility  by 
square-rigged  vessels  than  by  fore-and-aft,  and  by 
short  vessels  than  by  long  ones. 

The  flying  proa,  of  which  we  have  spoken,  does 
not  change  her  course  by  either  of  these  methods,  but 
does  it  partly  by  a  portion  of  the  manoeuvre  of  veer- 
ing and  partly  by  changing  the  position  of  the  sails, 
so  that  the  part  of  the  proa  which  was  before  the 
stern  becomes  the  bow.  This  method  is  practicable 
in  this  case,  because  both  ends  are  exactly  alike,  and 
the  mast,  which  is  stepped  upon  the  outrigger,  is 
equidistant  from  them. 

248.  Vessels,  generally  speaking,  sail  best  the 
more  nearly  they  retain  a  vertical  position.  Indeed, 
this  is  always  the  case,  except  when,  in  giving  them 
stability,  the  violence  of  rolling  is  increased.  Hence, 
in  increasing  the  area  of  the  sails  in  a  given  vessel, 
it  is  better  to  do  it  by  increasing  the  length  of  the 
yards  rather  than  that  of  the  masts,  for  the  action  of 


PRACTICAL  MECHANICS.         245 

a  wind  of  given  force  to  propel  the  vessel  depends 
upon  the  area  of  the  sails,  while  that  which  tends  to 
overturn  the  vessel  depends  on  the  distance  at  which 
the  disturbing  force  acts,  as  well  as  upon  this  area. 

249.  The  masts  of  vessels  ought  in  most  cases  to 
be  vertical,  leaning  neither  towards  the  bow  nor  to- 
wards the  stern.  In  very  sharp  vessels  they  may 
lean  towards  the  stern,  for  in  that  case  a  part  of  the 
force  will  be  exerted  to  prevent  the  vessel  from  bury- 
ing itself  too  deep  in  the  act  of  pitching.  When  the 
wind  acts  from  abaft,  its  action  will  generally  be  in 
part  exerted  to  depress  the  bow.  Hence  vessels  are 
usually  so  constructed  and  loaded  that  the  draught  of 
water  shall  be  less  at  the  bow  than  at  the  stern,  and 
nearly  all  vessels  sail  best  when  this  is  the  case. 

250.  When  vessels  are  propelled  by  steam,  the  ap- 
paratus which  is  now  used,  to  the  exclusion  of  all 
others,  is  a  paddle-wheel,  resembling,  in  its  general 
form  and  construction,  the  undershot  water-wheel. 
From  this  it  must,  however,  differ  in  the  number  of  its 
floats  ;  for  while  in  the  undershot  wheel  the  number 
of  floats  should  be  such  that  four  may  be  immersed  at 
one  time,  no  more  than  two  should  be  immersed  in 
the  paddle-wheel.  The  reason  of  this  difference  is, 
that  in  the  undershot  wheel  the  action  of  the  water 
will  be  increased  by  obstructing  its  flow  past  the 
wheel,  while  in  the  paddle-wheel  it  is  advantageous 
that  the  wheel  should  strike  against  water  which  has 
not  been  disturbed,  or  disturbed  as  little  as  possible, 
by  the  preceding  paddles. 

Paddles  placed  at  such  distances  from  each  other 
on  a  wheel  meet  with  a  resistance  from  the  water 
which  is  continually  varying.  The  motion  of  the 
wheel  would  thus  be  rendered  even  more  irregular 
than  it  would  under  the  varying  action  of  the  steam 
20 


246  PRACTICAL    MECHANICS. 

on  the  piston  of  the  engine.  The  action  of  the  wheel 
itself  as  a  fly  does  not  compensate  these  irregulari- 
ties. In  the  earlier  and  more  slowly-moving  steam- 
boats, it  was  therefore  found  expedient  to  make  use 
of  a  fly-wheel,  driven  with  a  velocity  greater  than 
that  of  the  paddle-wheel.  In  more  rapid  motions 
this  was  less  necessary  ;  and  a  motion  has  finally  heei 
obtained,  subject  to  no  greater  inequalities  than  the 
inertia  of  the  wheel  itself  is  capable  of  controlling. 
This  is  done  by  the  wheel  of  Stevens,  whose  con- 
struction may  be  understood  by  supposing  a  common 
paddle-wheel  to  be  sawn  into  three  equal  portions, 
divided  by  two  planes  perpendicular  to  the  axis,  and 
that  two  of  these  portions  are  moved  backward  until 
the  arc  intercepted  between  two  of  the  original  paddles 
is  divided  into  three  equal  parts.  The  number  of 
impulses  given  by  the  wheel  is  therefore  tripled,  and 
each  has  only  a  third  of  the  original  intensity.  The 
paddles  do  not  follow  in  each  other's  wake,  and  hence 
each  enters  into  water  which  has  been  but  little  dis- 
turbed by  the  preceding  paddles.  This  method  ap- 
pears to  possess  advantages  over  the  method  used  by 
the  English,  in  which  the  several  paddles  are  each 
divided  into  three  parts,  by  cutting  them  in  a  direc- 
tion parallel  to  the  face  of  the  wheel. 

251.  It  has  been  attempted  to  cause  the  paddle  to 
enter  the  water  in  a  vertical  position,  and  continue 
in  such  a  position  during  the  whole  time  of  its  im- 
mersion. This  attempt  has  been  founded  on  the  im- 
pression that  the  action  of  the  paddle  was  most  pow- 
erful under  such  circumstances.  Actual  observation 
has  shown  that  this  impression  is  incorrect ;  and 
Barlow  has  demonstrated  conclusively  that  the  max- 
imum of  the  eifect  of  a  common  paddle-wheel  does 
not  occur  when  the  paddle  is  vertical. 

The  theory  of  the  action  of  paddle-wheels  has  not 


PRACTICAL  MECHANICS.         247 

been  satisfactorily  solved  by  analysts.  This  is  part. 
ly  owing  to  the  imperfection  we  have  remarked  in 
the  theory  of  the  engine  itself,  which  has  not  hitherto 
had  any  regard  to  the  difference  of  pressure  of  the 
steam  upon  a  piston  when  at  rest  and  when  in  mo- 
tion, and  partly  to  the  want  of  a  satisfactory  ana- 
lytic expression  for  the  action  of  the  wheel  on  the 
water.  The  theory  has  therefore  led  to  a  conclu- 
sion which  is  contradicted  by  all  the  facts,  and  in  the 
hands  of  Tredgold  to  one  which  is  manifestly  absurd. 
The  first  of  these  conclusions  is,  that  the  relation 
between  the  velocity  of  the  vessel  and  that  of  the 
wheel  is  a  constant  quantity,  or  may  be  expressed  by 
the  formula 

V 

—  =  m. 

V 

The  fact,  obtained  by  a  comparison  of  the  rates  ot 
many  vessels,  and  of  the  same  vessel  moved  with 
powers  varying  in  the  ratio  of  2  :  1,  is,  that  the  dif- 
ference between  these  two  velocities,  or  the  relative 
velocity  of  the  circumference  of  the  wheel  is,  in 
wheels  of  similar  form,  a  constant  quantity.  This 
fact  may  be  represented  by  the  expression 

The  absurdity  into  which  Tredgold  has  been  led 
is  the  statement  that  the  velocity  of  a  given  vessel 
through  the  water  will  be  different  according  as  it 
moves  with  or  against  the  current.  Now  if  we  con- 
sider  a  vessel  to  be  in  the  first  place  abandoned  to 
the  stream,  it  will  speedily  acquire  the  velocity  of  the 
current,  and  be  at  rest  relatively.  If,  now,  the  en- 
gine  be  set  in  motion,  the  velocity  in  relation  to  the 
water  will  be  the  same  in  any  direction  whatever, 
and  the  same  result  will  be  reached,  even  if  the  ves- 


248  PRACTICAL    MECHANICS. 

sel  be  set  in  motion  on  leaving  a  fixed  fastening  to 
the  shore,  by  which  a  current  is  moving.  The  ve- 
locity, which  we  have  stated  as  constant  in  similar 
wheels,  varies  in  different  wheels,  according  to  the 
depth  they  are  immersed  in  the  water,  and  the  num- 
ber of  paddles  which  occur  on  their  circumference, 
from  6.2  feet  to  6.8  feet  per  second. 

252.  In  the  usual  theory  it  is  assumed  that  the  re- 
sistance is,  in  all  cases,  proportioned  to  the  squares 
of  the  velocities.  Hence  the  power  of  the  engine 
ought  to  vary  with  the  cube  of  the  velocity,  and  the 
expenditure  of  fuel,  in  passing  through  a  given  dis- 
tance, with  the  squares  of  the  velocities.  The  expe- 
rience of  American  engineers  seems  to  prove  that 
this  assumption  is  not  true  at  the  higher  velocities. 
They  have  found  that  every  increase  in  the  rate  of 
the  revolution  of  the  wheel,  has  been  attended  with 
an  increase,  not  proportioned  to  it,  but  absolutely 
equal,  in  the  velocity  of  the  vessel.  If  this  be  true : 
the  measure  of  the  resistance  to  a  given  vessel  is 
constant,  like  friction ;  the  expenditure  of  fuel  in  a 
given  time  should  be  as  the  velocity,  and,  in  passing 
through  a  given  distance,  constant.  The  nominal 
power  of  the  engines  should  increase  simply  with  the 
velocity,  and  the  increase  in  the  action  of  the  prime 
mover  be  applied  to  increase  velocity  only.  We  are 
aware  that  these  principles  appear  startling  to  Euro- 
pean engineers  and  mathematicians,  and  have  already 
undergone  their  censure  for  having  stated  them.  Ac- 
tual experiment  can  alone  decide  between  us. 

In  the  mean  time,  we  may  cite  European  experi- 
ence  in  relation  to  the  motion  of  boats  on  a  canal, 
which  in  some  measure  bear  out  our  conclusions. 

In  experiments  made  on  the  Paisley  Canal,  two 
horses  drew  a  passage -boat,  weighing,  with  her  load, 
3 J  tons,  at  the  rate  of  10.383  miles  per  hour,  and 


PRACTICAL   MECHANICS.  249 

exerted  a  force  in  draught,  measured  by  the  dyna- 
mometer, of  285.15  lbs.  Now,  taking  for  a  basis  of 
calculation  that  a  horse,  exerting  a  force  in  draught 
of  186|  lbs.  is  capable  of  drawing  a  weight  of  30  tons 
with  a  velocity  of  2J  miles  per  hour,  the  force  ne- 
cessary to  draw  3^  tons  at  the  above  velocity,  if  the 
resistance  increased  with  the  square  of  the  velocity, 
o'lfrht  to  have  been  746.6  lbs.  Up  to  this  limit,  how- 
ever,  the  resistance  increases  in  a  ratio  greater  than 
that  of  the  simple  velocity  ;  for,  calculated  on  that  hy- 
pothesis, the  measure  should  have  been  no  more  than 
190  lbs. 

It  is  obvious  that  much  remains  to  be  done,  not 
only  in  the  theory  of  the  application  of  steam  to  nav- 
igation, but  in  the  investigation  of  the  experimental 
laws  which  serve  as  its  basis ;  and  it  is  unfortunate 
that  no  experiments  have  yet  been  made  at  veloci- 
ties greater  than  the  least  which  are  now  ever  given 
to  steamboats.* 

♦  According  to  the  theory  which  is  now  usually  received : 

(1.)  The  force  required  to  move  vessels  of  similar  figures  with 
equal  velocities,  varies  with  the  squares  of  their  homologous  di- 
mensions.   This  proposition  alone  is  unquestionably  true. 

(2.)  The  relative  velocity  of  the  wheels  ought  to  be  increased 
with  the  diminution  of  the  relation  of  their  areas  to  that  of  the 
midship  section  of  the  vessel.  This  cannot  be  effected  in  practice, 
because  this  relative  velocity  is  constant. 

(3.)  The  power  of  the  engine  must  vary  with  the  area  of  the 
midship  section,  with  the  cube  of  the  velocity,  and  with  the  square 
root  of  the  relation  between  the  midship  section  and  the  paddles, 
augmented  by  unity. 

(4.)  The  velocity  is  directly  proportioned  to  the  cube  root  of  the 
power  of  the  engine,  and  inversely  to  the  cube  root  of  the  area  of 
the  midship  section. 

(5.)  From  these  laws  an  expression  has  been  deduced  for  the 
velocity  to  be  attained  in  the  average  of  vessels  by  a  given  power 
of  engine.    This  is 

V— 2?/—  • 

F  being  the  number  of  horse  powers,  6  the  breadth,  an4  d  the 
draught  of  water. 


250  PRACTICAL    MECHANICS. 

253.  In  the  construction  of  steam-vessels,  which 
are  not  intended  to  navigate  the  ocean,  the  only  point 
which  is  of  importance  in  their  models  is  to  give  them 
the  figure  of  least  resistance  ;  and  the  most  impor- 
tant part  of  this  resistance  is  the  wave  which  tends 
to  be  raised  in  front  of  the  vessel,  taken  along  with 
the  depression  which  is  caused  in  the  wake.  Our 
builders  have  sought  this  figure  by  continual  experi- 
ment and  observation,  adapting  false  prows  and  sterns 
to  the  vessels,  and  noting  the  effects  they  produced. 
The  practice  derived  from  this  course  of  experiment 
is  so  successful,  that  there  are  vessels  in  our  waters 
which  throw  up  no  sensible  wave,  and  leave  no  de- 
pression behind  them.  This  was  the  case  with  the 
New-York,  which  was  recently  destroyed  by  fire. 
In  sea-going  vessels,  this  advantage  must  probably  be 
sacrificed,  in  order  to  obtain  others  which  are  of  more 
moment.  These  are  strength,  stability,  and  security 
from  the  injuries  produced  by  pitching  and  rolling 
under  the  action  of  the  waves.  As  yet,  at  least,  the 
qualities  of  speed  and  safety  at  sea  have  not  been 
found  compatible. 

Supposing  that  it  were  true  that  there  is  a  constant 
relation  of  the  velocities  of  the  vessel  and  the  wheel, 
the  maximum  velocity  of  a  steam-vessel,  if  that  sta- 
ted for  the  paddle  be  the  limit,  would  be  about  12 
nautical  miles  per  hour.  Our  American  steam- ves- 
sels, however,  have  considerably  exceeded  this  the- 
oretic maximum. 

254.  An  engine,  of  the  structure  which  has  been 
described,  requires  to  be  modified  before  it  can  be 
used  in  navigation.  The  cold-water  cistern  is  dis- 
pensed with  as  lessening  the  buoyancy  of  the  vessel. 
In  lieu  of  this,  the  condenser  has  been  increased  in 
volume  four  fold.  The  water  of  condensation  is  in- 
troduced into  the  condenser  by  means  of  a  pipe  pass- 


PRACTICAL    MECHANICS.  251 

ing  through  the  bottom  of  the  vessel,  in  which  the  wa- 
ter rises  as  high  as  the  water  without.  The  surplus 
of  water  from  the  hot- water  cistern  is  discharged 
through  a  similar  pipe.  The  boiler  can  no  longer 
be  set  in  masonry  ;  hence  the  furnace  is  formed  by 
the  metallic  surfaces  of  the  boiler,  and  the  flues  pass 
through  it. 

In  American  steamboat  engines,  the  connecting- 
rod  has  been  lengthened,  and  thus  acts  with  less  ob- 
liquity ;  the  stroke  of  the  piston  has  been  lengthened 
also,  and  thus  the  crank  is  made  to  act  upon  the 
wheel  at  a  more  favourable  point ;  for  it  is  obvious, 
that  it  is  only  when  it  acts  at  a  distance  from  the 
axis  of  the  wheel,  which  is  equal  to  that  of  the  centre 
of  pressure  on  the  paddle,  that  the  two  forces  exactly 
counterbalance  each  other,  and  that  none  of  the  force 
of  the  prime  mover  is  wasted  upon  the  axle  of  the 
wheel.  It  would  be  impracticable  to  make  the  crank 
of  sufficient  length  to  attain  this  most  advantageous 
position,  but  the  more  nearly  it  approaches  to  it  the 
better. 

In  the  English  engines,  from  a  desire  to  keep  the 
weight  as  low  as  possible,  the  comparative  length  of 
the  stroke  has  been  lessened.  The  crank  is  there- 
fore applied  to  a  point  which  is  less  favourable.  In 
the  American  method,  too,  it  has  been  possible  to 
give  a  greater  mean  velocity  of  the  piston,  because 
the  dead  points  recur  less  frequently. 

In  order  to  secure  this  last  advantage,  the  area  of 
the  steam-pipes  and  of  the  valves  has  been  much  in- 
creased ;  thus  the  velocity  of  the  pistons  of  the 
American  boat  engines  has  been  raised,  from  the  old 
limit  of  little  more  than  200  feet  per  minute,  to  near- 
ly 600  feet. 

255.  Paddle-wheels  cannot  be  used  upon  canals, 
in  consequence  of  the  great  agitation  they  produce 


252         PRACTICAL  MECHANICS. 

in  the  water,  and  consequent  injury  to  the  banks.  It 
has  been  proposed  in  France,  by  Tourasse,  to  apply 
steam  to  vessels  upon  canals  by  means  of  a  chain 
extending  along  the  bottom  from  lock  to  lock.  This 
is  lifted,  and  wound  around  a  barrel  placed  on  the  axle 
of  the  crank.  By  the  revolution  of  the  axle,  the  chain 
is  drawn  in  from  the  bow  of  the  boat,  and  discharged 
in  the  opposite  direction.  In  this  way,  the  whole 
force  of  the  steam-engine  will  be  employed  in  the 
draught.  The  great  objection  to  the  method  lies  in 
the  original  cost  of  the  chains.  In  passing  short  dis- 
tances, as  upon  a  ferry,  this  objection  does  not  apply, 
and  this  plan  has  been  recently  successfully  used 
upon  a  ferry  in  England. 

We  have  recently  seen  a  great  improvement  on 
this  method,  by  Mr.  Leavenworth,  of  New-York.  He 
has  ascertained,  that  the  mere  friction  of  a  chain  on 
the  bottom  of  a  canal  is  sufficient  to  propel  a  boat 
along  it.  He  therefore  makes  use  of  an  endless 
chain,  extending  from  near  the  bows  almost  to  the 
stern  of  the  boat.  This  is  set  in  motion  by  an  axle 
moved  by  the  steam-engine,  and  while  one  branch  of 
the  chain  is  carried  along  with  the  boat,  the  other 
drops  on  the  bottom  of  the  canal.  This  apparatus 
has  been  used  with  success  upon  the  Morris  Canal. 


PRACTICAL   MECHANICS.  253 

XL 

MACHINES   USED    IN   MANUFACTURES. 

256.  Machines  used  in  manufactures  may  be 
propelled  by  any  of  the  great  natural  agents.  The 
force  of  men,  of  animals,  and  of  the  wind,  have  been 
all,  and  are  still  occasionally  employed ;  but  water 
acting  upon  wheels,  and  the  steam-engine,  are  better 
suited  to  all  cases  in  which  regularity  and  perma- 
nency of  action  are  required.  Up  to  the  present 
time,  the  force  of  water  is  regarded  in  the  most  fa- 
vourable light  in  the  United  States,  but  it  may  be 
questioned  whether  this  preference  be  well  founded. 
In  those  districts  of  our  country  where  there  is  at 
present  a  surplus  population  applicable  to  manu- 
facturing purposes,  fuel  is  dear,  and  water-power  is 
abundant ;  it  is  therefore  considered  as  the  most 
economic.  But  water  is,  at  best,  an  uncertain  power  ; 
the  machinery  may  be  prevented  from  working,  not 
only  in  seasons  of  drought,  but  by  the  fulness  of  the 
streams ;  the  dams  and  races  by  which  the  power 
is  supplied  are  liable  to  injury  and  destruction  by 
floods.  Water-power  must  also  be  sought,  and  the 
manufacturer  must  leave  all  other  considerations 
out  of  view  in  choosing  a  site  for  his  establishment. 
We  have  seen  calculations  founded  on  actual  facts, 
by  which  it  has  been  shown  that,  taking  all  things 
into  account,  the  actual  cost  of  cotton  goods  manu- 
factured by  steam  in  the  city  of  New- York,  is  less 
than  that  of  similar  articles  manufactured  in  Patter- 
son by  water-power. 

257.  The  velocity  with  which  the  fly  of  a  steam- 


254  PRACTICAL    MECHANICS. 

engine  or  an  overshot  wheel  revolves,  in  order  to  do 
the  greatest  quantity  of  work,  may  be  considered  as 
fixed.  The  velocities  with  which  differem  descrip- 
tions of  work  are  performed  are  each  fixed,  but  are 
never  the  same,  either  in  their  direction  or  their 
rate,  as  those  best  adapted  to  the  favourable  action 
of  the  prime  mover.  It  therefore  becomes  necessa- 
ry to  change  the  motion  of  the  working  point  of  the 
machine  in  its  direction  and  intensity,  for  the  pur- 
pose of  performing  the  desired  operation  in  a  proper 
manner.  These  clianges  are  principally  performed 
by  combinations  of  the  wheel  and  axle,  in  its  several 
modifications. 

258.  One  wheel  may  turn  another,  or  a  wheel 
may  turn  an  axle,  by  the  direct  friction  of  their  re- 
spective circumferences.  There  are  some  cases  in 
which  the  cyUndrical  surface  of  one  wheel  is  thus 
made  to  turn  another,  but  they  are  rare.  The  dif- 
ficulty of  using  this  method  consists  in  the  risk  of 
sliding,  and  this  can  only  be  obviated  by  introducing 
X  degree  of  friction  which  would  render  a  combina- 
lion  of  this  sort  inefficient.  Nor  would  even  a  great 
friction  answer  the  purpose,  unless  the  velocity  of  the 
wheels  and  their  mutual  pressure  were  constant ;  but 
when,  as  is  most  frequently  the  case,  the  moving 
power  or  the  resistance  acts  with  variable  intensity, 
',1  would  be  impossible  to  avoid  the  sliding. 

259.  The  second  mode  in  which  wheels  and  axles 
may  be  combined  with  each  other,  is  through  the  in- 
tervention of  bands,  composed  of  ropes,  straps,  or 
chains.  These  are  also  liable  to  slide;  but  this 
tendency  is  not  always  disadvantageous,  although 
always  attended  with  a  loss  of  moving  power,  for 
the  sliding  of  the  band  may  prevent  any  risk  of  frac- 
ture in  the  parts  of  the  machine  or  of  the  engine, 


PRACTICAL  MECHANICS.         255 

when  the  resistance  is  liable  to  sudden  changes  in 
its  intensity. 

The  use  of  bands  is  principally  confined  to  the 
case  where  the  motion  is  to  be  transmitted  to  a  dis- 
tance. When  ropes  are  employed,  a  groove  must 
be  cut  in  the  circumference  of  the  wheels,  as  in  the 
pulley,  and  they  may  be  made  to  change  the  plane 
in  which  the  motion  is  performed. 

When  bands  are  employed,  the  surface  of  the 
wheels  should  be  slightly  curved.  Flat  chains 
should  only  be  employed  when  the  tension  to  which 
they  are  subjected  is  small,  and  the  friction  is  not 
regarded.  In  other  cases,  chains  formed  like  those 
of  a  watch,  or  made  in  links  of  the  usual  descrip- 
tion, are  caught  upon  teeth  placed  on  the  circumfer- 
ence of  the  wheels.  The  chain  must  be  so  long 
that  its  returning  branch  shall  be  slack. 

The  tension  of  bands  often  requires  to  be  main- 
tained by  some  extrinsic  action,  for  it  would  not  do 
to  draw  them  too  tight,  in  consequence  of  the  great 
increase  of  friction  which  would  be  thus  caused. 
The  best  mode  of  giving  the  proper  degree  of  ten- 
sion is  by  allowing  a  heavy  wheel  to  rest  on  the 
bands,  the  axle  of  which  is  connected  by  a  radius 
bar  with  a  firm  support.  When  thus  loaded,  the 
same  band  may  be  shifted  from  one  axle  to  another 
of  different  diameter,  while  it  is  driven  by  a  wheel 
of  constant  diameter,  and  different  velocities  may 
thus  be  given  to  the  common  axis  of  the  several 
axles. 

One  of  the  most  difficult  cases  in  the  use  of  bands 
to  connect  wheels  and  axles,  is  that  in  which  it  was 
wished  to  turn  the  potter's  wheel  by  machinery,  pro- 
pelled by  water  or  steam,  instead  of  using  human 
labour.  So  difficult  was  this  considered,  that,  after 
it  had  for  many  years  been  attempted  in  vain,  it  was 


256  PRACTICAL    MECHANICS. 

almost  abandoned  as  impracticable.  Finally,  the 
desired  effect  was  obtained  in  the  simplest  possible 
manner ;  the  band  was  passed  over  two  cones, 
whose  points  or  vertices  were  turned  in  opposite  di- 
rections. Thus,  when  the  band  was  passed  over  the 
middle  of  the  cones,  the  velocities  of  both  were 
equal,  while,  by  moving  it  towards  one  or  the  other 
end,  every  desired  variety  in  the  rate  of  the  motions 
could  be  obtained. 

260.  The  third  mode  in  which  wheels  and  axles 
are  combined,  is  by  cutting  their  circumferences 
into  teeth  which  catch  into  each  other.  The  prin- 
ciples on  which  these  toothed  wheels  and  axles  act, 
are  a  part  of  the  theory  of  mechanics,  and  for  them 
we  refer  to  our  Treatise  on  that  subject.*  It  is 
there  stated  that  there  are  two  principal  modifi- 
cations of  toothed  wheels  and  axles,  known  under 
the  names  of  the  wheel  and  trundle,  and  the  wheel 
and  pinion.  In  the  case  of  the  wheel  and  trundle, 
a  motion  may  be  taken  off  at  right  angles  by  ma- 
king the  teeth  of  the  wheel  at  right  angles  to  its 
surface  ;  these  will  adapt  themselves  to  the  staves 
of  a  trundle  whose  axis  is  also  at  right  angles  to 
that  of  the  wheel.  Motions  of  small  obUquity  may 
be  taken  off  by  means  of  the  universal  joint  of  Hook. 
To  construct  this,  the  axle  of  the  wheel  is  forked  in 
the  form  of  a  stirrup-iron  at  its  extremity ;  another 
axle,  lying  in  the  desired  direction,  has  its  extremity 
of  the  same  figure ;  the  two  axles  are  united  by  a 
cross,  the  ends  of  whose  arms  are  turned,  and  thus 
formed  into  gudgeons,  which  rest  in  circular  holes  in 
the  forked  branches  of  the  two  axles. 

When  a  trundle  is  applied  to  turn  a  wheel,  it  be- 
gins to  act  before  the  touching  surfaces  reach  the 

*  Treatise  on  Mechanics,  book  ill.,  chap. 


PRACTICAL   MECHANICS.  257 

line  which  joins  the  centres,  the  friction  is  thus  ren- 
dered great  and  the  motion  harsh.  Even  when  the 
wheel  turns  the  trundle,  the  combination  is  attended 
with  inconvenience,  from  the  harshness  of  the  mo- 
tion, and  the  great  wear  to  which  the  staves  of  the 
trundle  are  subjected.  To  remedy  this,  the  staves 
are  usually  made  of  iron,  and  the  teeth  of  the  wheel 
of  wood ;  but  the  latter  material  is  so  soft  that  the 
inequality  of  wear  is  thus  thrown  upon  it.  The 
wheel  and  trundle  is,  however,  so  cheap  and  simple 
in  its  construction,  that  it  is  not  wholly  abandoned. 

261.  In  the  use  of  the  wheel  and  pinion,  motion 
may  be  taken  off  at  right  angles,  by  cutting  the  teeth 
of  the  wheel  into  the  envelope  of  a  hollow  cylinder. 
This  method  has  been  described  in  speaking  of  the 
construction  of  the  watch,  under  the  name  of  the 
contrate  wheel.  In  heavier  machinery,  motion  at 
any  desired  angle  may  be  taken  off  by  cutting  the 
teeth  of  the  wheels  or  pinions  which  act  upon  each 
other  upon  the  surfaces  of  two  cones,  whose  vertices 
meet  in  the  same  point,  and  whose  axes  make  with 
each  other  the  required  angle  of  obliquity.* 

*  In  the  construction  of  wheels  and  pinions  it  is  necessary : 

(1.)  That  the  number  of  their  respective  teeth  shall  be  in  the 
Inverse  ratio  of  their  respective  angular  velocities. 

(2.)  That,  if  two  circles  be  drawn  concentric  with  the  wheels, 
and  tangent  to  each  other,  whose  radii  are  in  the  inverse  ratio  of 
their  respective  angular  velocities,  the  arcs  intercepted  between 
the  middle  points  of  the  consecutive  teeth  on  these  two  circles 
shall  be  equal  to  each  other ;  that  is  to  say,  not  only  shall  such 
circumference  be  divided  into  parts  exactly  equal,  but  the  parts 
on  the  two  separate  circles  shall  also  be  equal  to  each  other;  these 
circles  are  called  the  pitch-lines  of  the  wheels. 

(3.)  The  curved  surfaces  of  the  teeth  must  be  so  constructed 
that  the  uniform  velocity  of  the  driving  wheel  shall  be  communi- 
cated to  the  other,  during  the  whole  time  of  the  contact  of  any 
two  of  their  respective  teeth. 

For  this  purpose  it  is  necessary  so  to  arrange  the  thickness,  the 
projection,  and  the  intervening  spaces  of  the  teeth,  that  a  giver 


258  PRACTICAL   MECHANICS. 

tooth  shall  begin  to  act  at  the  instant  when  that  which  precedes 
it  is  ceasing  to  touch  the  corresponding  tooth  on  the  other  wheel. 

It  is,  moreover,  important  that  no  teeth  shall  begin  to  act  upon 
each  other  until  their  surfaces  are  respectively  in  the  line  which 
joins  the  two  centres.  In  this  case  the  friction  is  less,  and  the 
risk  to  the  v.'heels  from  fracture  is  lessened  also.  The  opposite 
surfaces  of  a  given  tooth  are  to  be  exactly  alike,  in  order  to  pro- 
vide for  the  reversal  of  the  motion. 

The  least  projection  of  the  teeth  beyond  the  pitch-lines  of 
the  wheels,  must  be  such  that  one  tooth  shall  arrive  in  the  line 
which  joins  the  centres  of  the  circles  at  the  instant  the  tooth 
which  precedes  it  ceases  to  act ;  and  the  depth  of  the  cavities  be- 
low the  pitch-line,  must  be  such  as  will  admit  the  free  motion  of 
the  teeth  of  the  other  wheel. 

The  thickness  and  size  of  the  teeth  will  depend  upon  the  rela- 
tion between  the  strength  of  the  material  of  which  they  are  com- 
posed and  the  strain  to  which  they  are  subjected  ;  and  there  must 
be  a  sufficient  space  between  them  not  only  to  admit  the  teeth  of 
the  other  wheel,  but  to  allow  a  certain  degree  of  play,  to  compen- 
sate for  any  imperfection  in  material  or  workmanship.  This  play, 
even  in  good  workmanship,  amounts  to  1-lOth  or  l-12th  of  the 
thickness  of  the  tooth,  and  ought  generally  to  be  more. 

The  teeth  are  bounded  on  each  side  by  radii  of  the  pitch-line 
reaching  as  far  as  the  circumference  of  that  circle  from  the  bottom 
of  the  cavity,  and  the  cavity  terminates  in  an  arc  of  a  circle  concen- 
tric with  the  pitch-line.  From  the  pitch-line  outward  the  proper 
figure  is  a  portion  of  an  epicycloid,  described  upon  the  pitch-line  by 
the  revolution  of  a  circle  whose  diameter  is  half  the  pitch  of  the 
other  wheel.  These  cycloidal  arcs  on  the  opposite  sides  of  the 
tooth,  should  be  cut  off  by  a  plane  surface,  at  such  a  distance  from 
the  pitch-line  as  will  prevent  the  tooth  from  continuing  to  act  af- 
ter that  which  precedes  it  reaches  the  line  of  the  centres. 

In  practice,  epicycloidal  curves  are  difficult  to  construct ;  they 
are  therefore  replaced  by  circular  arcs.    The  usual  mode  of  con* 
etmction  may  be  understood  from  fig.  81. 
Fig.  81. 


PRACTICAL    MECHANICS.  259 

262.  In  giving  strength  to  the  teeth  of  a  wheel,  it 
is  better  to  increase  the  thickness  of  the  wheel,  and 
thus  the  length  of  the  teeth,  than  the  thickness  of 
the  teeth  themselves.  It  is  also  better  to  make  the 
wheels  of  large  diameter,  as  thus  the  power  acts 
upon  a  longer  lever  to  overcome  the  friction.  In 
this  way  also  the  number  of  teeth  is  increased,  while 
the  relation  of  those  on  the  two  wheels  remains  the 
same,  and  a  more  equable  action  is  produced.  In  a 
pinion  which  has  no  more  than  eight  teeth,  each 
tooth  begins  to  act  before  it  reaches  the  line  of  the 
centres,  and  cannot  be  disengaged  as  soon  as  the 
following  tooth  begins  to  act.  A  pinion  of  ten  teeth 
will  not  have  the  proper  qualities  if  driven  by  a 
wheel  of  less  than  72  teeth  ;  but  in  all  higher  num- 
bers there  is  no  difficulty.  Pinions  of  less  than  six 
teeth  should  never  be  used. 

263.  The  principle  on  which  the  action  of  wheels 
and  pinions  rests,  is  as  follows  :*  The  power  is  to  the 
resistance  as  the  continued  product  of  the  number  of 
teeth  on  all  the  pinions  is  to  the  continued  product  of 
the  teeth  of  all  the  wheels.  A  train  of  pinions  dri- 
ving wheels  is  therefore  a  powerful  means  of  in- 
creasing the  intensity  of  a  force,  while,  if  its  action 

A  C  B  is  the  pitch-line  of  the  wheel,  on  which,  after  dividing 
it  into  as  many  equal  parts  as  it  is  intended  to  form  teeth,  the  half 
breadth  of  each  tooth  is  set  oif  on  each  side  from  the  points  of  di- 
vision, as  from  C  to  d  and  e.  From  the  points  d  ana  e  radii  are 
drawn  until  they  meet  ttie  circle  F  G,  by  v^hich  the  cavities  be- 
tween the  teeth  are  bounded.  The  curved  faces  of  the  teeth  are 
formed  by  describing  circles  around  the  dividing  points  of  the  cir- 
cle, as  around  C,  as  centres,  with  radii  equal  to  the  distance  from 
such  point  to  the  opposite  face  of  the  contiguous  tooth.  This  ra- 
dius is  equal  to  the  cord  of  1^  of  the  divisions  into  which  the  pitch- 
hne  has  been  divided.  The  points  formed  by  the  intersecting  arcs 
of  these  circles  are  then  cut  off*  by  lines,  h  i,  parallel  to  the  tan- 
gents of  the  pitch-line. 

*  See  Treatise  on  Mechanics,  book  iii.  chap  vi. 


260         PRACTICAL  MECHANICS. 

be  reversed,  and  wheels  are  made  to  drive  pinions, 
the  velocity  is  increased  in  the  same  proportion. 
Of  the  latter  case  we  have  had  instances  in  the 
s:ructure  of  the  clock  and  watch. 

The  direct  problem  of  finding  the  relative  times 
of  revolution  of  two  wheels  or  pinions  belonging  to 
the  same  train,  is  an  easy  and  obvious  application 
of  the  above  principle.  That  of  finding  the  proper 
number  of  wheels  and  pinions  to  be  interposed,  and 
the  number  of  teeth  upon  each,  is  one  of  greater 
difficulty,  and  every  complex  case  may  admit  of  a 
variety  of  solutions.  The  mode  of  proceeding  can 
be  best  illustrated  by  an  example  taken  from  one  of 
the  forms  of  clock.* 


*  Suppose  it  to  be  wished  that  a  wheel  which  revolves  in  2^  days 
shall  give  motion  to  another  whose  revolution  shall  be  performed 
in  29i  days.  The  number  of  minutes  in  the  first  interval  is  3600, 
and  in  the  second  42,524 ;  and  any  numbers  having  the  same  rela- 
tion as  these  will  represent  the  number  of  teeth  which,  if  applied 
to  two  wheels,  will  answer  the  purpose.  Both  numbers  being 
divisible  by  4,  900  :  10,631  will  express  this  relation.  But  so  great 
a  number  of  teeth  as  either  of  these  cannot  be  constructed  upon 
wheels  and  the  larger  of  the  two  is  a  prime  number,  which  can- 
not be  decomposed  into  factors,  for  the  purpose  of  using  these 
factors  as  the  numbers  of  teeth  on  intermediate  wheels.  Instead 
of  these  numbers,  then,  two  others  are  chosen,  having  nearly  the 
same  relation,  namely,  703  : 8.304.  These  numbers  are  each  de- 
composed into  two  factors,  thus:  703=19x37,  8304=48x173. 
If,  then,  a  train  of  wheels  be  composed  as  follows  :  let  a  pinion  of 
19  teeth  drive  a  wheel,  B,  of  48  teeth ;  place  a  pinion,  6,  on  the 
same  axle  with  B  ;  give  it  37  teeth,  and  let  it  drive  a  wheel  of  173 
teeth,  the  desired  result  will  be  obtained  nearly.  This  is  the  case 
which  is  employed  in  clocks  intended  to  show  the  phases  of  the 
moon,  and  it  so  happens  that  the  relation  703  :  8304  is  more  near 
to  that  which  the  lunation  bears  to  the  solar  day,  than  the  one 
originally  assumed. 

Calculations  of  this  sort  are  of  most  mterest  when  it  is  m- 
tended,  by  means  of  clockwork,  to  represent  the  planetary  mo- 
tions. The  best  instance  of  this  sort  is  the  orrery  constructed 
by  Rittenhouse,  a  description  of  which  may  be  seen  in  the  life  of 
that  distinguished  artist  and  astronomer,  in  Sparks's  American  Bi- 
ographyv 

One  of  the  best  instances  of  a  combination  of  wheels  and  pin* 


PRACTICAL  MECHANICS. 


261 


ions  to  obtain  an  increase  in  the  inten- 
sity of  a  force,  is  to  be  found  in  the 
machine  for  proving  chain-cables. 

In  this  machine  an  axle  of  five  inch- 
es in  diameter,  over  which  the  chain 
is  wound,  is  united  to  a  wheel  of  two 
feet  in  diameter ;  this  wheel,  F,  has 
73  teeth,  and  is  driven  by  a  pinion  of 
nine  teeth ;  the  latter  is  on  the  same 
axis  with  a  wheel,  H,  of  97  teeth,  driv- 
en by  a  pinion  of  eight  teeth  ;  the  lat- 
ter is  on  the  same  axis  with  a  wheel,  I, 
of  97  teeth,  which  is  driven  by  a  pinion 
of  eight  teeth;  this  pinion  is  on  the 
same  axis  as  the  wheel  K,  of  97  teeth, 
which  is  driven  by  the  pinion  L,  of 
eight  teeth.  The  last  pinion  is  two 
inches  in  diameter,  and  is  turned  by 
a  winch,  whose  arm  is  14  inches. 
Leaving  out  the  odd  tooth  on  the  sev- 
eral wheels,  inserted  for  the  purpose 
of  rendering  the  wear  more  equable, 
the  relation  of  the  power  to  the  weight 
.    72     96     96      96      14      24      24 

^2^2       5      2^.3.5         ' 

If,  then,  a  man,  in  turning  a  winch, 
exert  a  force  of  30  pounds,  and  an  al- 
lowance of  one  third  be  made  for  fric- 
tion, he  will  act  to  stretch  the  cable 
submitted  to  the  machine  with  a  force 
equivalent  to  a  weight  of  36  tons. 

The  cable  to  be  proved  is  stretched 
over  a  frame.  A,  and  attached  to  the 
end  of  a  curved  lever,  B ;  the  longer 
arm  of  the  lever  B  is  acted  upon  by  the 
shorter  arm  of  a  lever,  C,  from  whose 
longer  arm  a  weight  is  suspended. 


Fig.  82. 


21 


262  PRACTICAL   MECHANICS. 


Flouring  Mills, 


264.  Mills  are  used  for  the  purpose  of  grinding 
grain  into  the  form  of  flour  or  meal,  in  which  they 
are  employed  in  the  manufacture  of  bread.  Origi- 
nally moved  by  the  force  of  men  or  animals,  they 
are  still  in  many  countries  driven  by  wind.  All 
these  prime  movers,  however,  are  so  irregular  in 
their  action,  that  they  are  ill  adapted  to  the  purpose, 
for  it  is  necessary  that  the  work  should  be  performed 
at  a  velocity  which  is  confined  within  very  narrow 
limits ;  at  less  velocities  the  grain  is  cut  instead  oi 
being  ground,  and  at  greater,  it  is  so  much  heated 
that  it  is  liable  to  ferment  in  spite  of  any  precaution. 

The  manufacture  of  wheaten  flour  was  for  many 
years  among  the  most  important  objects  of  our  na- 
tional industry.  It  was  almost  the  sole  staple  for  ex- 
port of  the  Middle  States,  and  was  carried  on  by 
merchant-millers  of  great  capital,  residing  in  the  prin- 
cipal commercial  cities.  In  the  change  which  has 
taken  place  in  the  occupations  of  our  countrymen, 
by  which  we  have  almost  ceased  to  export  breadstuflT,  « 
in  consequence  of  the  great  manufacturing  population;  I 
which  has  been  created,  the  course  of  this  trade  has  ^^ 
been  altered  ;  and  although  it  has  not  ceased  to  be  a 
favourite  object  of  commercial  enterprise,  the  posi- 
tion of  the  mills  has  been  changed.  The  merchant- 
millers  have  been  compelled  to  place  themselves  in 
the  neighbourhood  of  the  districts  where  an  excess 
of  breadstufl"  is  produced,  and  many  of  the  great  es- 
tablishments in  the  vicinity  of  the  seaboard  have  gone 
to  decay.  The  agriculturist  has  reaped  this  advan- 
tage, namely,  that  the  prices  of  his  products  are  now 
rather  regulated  by  the  cost  of  importing  a  similar 
article  from  abroad,  than  by  the  price  at  which  it  can 
be  safely  shipped  to  a  foreign  market. 


PRACTICAL   MECHANICS.  263 

During  the  time  that  wheaten  flour  was  a  great 
staple  for  export,  the  application  of  capital  and  in- 
genuity led  to  great  improvements  in  the  gristmill. 
Labour-saving  machinery  was  introduced  to  such  an 
extent,  as  to  leave  little  or  nothing  for  man  to  do,  ex- 
cept to  set  it  in  action  ;  and  the  mode  of  cleansing 
the  grain  and  the  meal  from  all  impurities  and  prod- 
ucts of  inferior  value,  was  brought  to  absolute  per- 
fection. The  most  perfect  mills  of  this  description 
in  the  world  are  probably  those  of  Rochester,  N.  Y. 
Those  on  the  Brandywine  and  at  Richmond,  Va., 
have  also  long  been  in  high  repute. 

265.  The  usual  grinding  apparatus  in  a  gristmill 
is  composed  of  two  circular  stones.  The  lower  one 
is  fixed  ;  the  upper  revolves  on  a  vertical  axis,  which 
passes  through  the  lower  and  is  attached  to  the  up- 
per  by  a  cross  of  iron,  whose  arms  are  sufficiently 
long  to  allow  of  a  hole  in  the  middle  of  the  upper 
millstone,  by  which  the  grist  is  admitted  to  the  space 
between  the  stones.  The  opposite  surfaces  of  the 
two  stones  are  divided  into  sectors,  and  these  sec- 
tors are  cut  into  groves,  parallel  to  one  of  the  radii 
which  bound  each  sector.  The  cuts  into  the  two 
millstones  are  precisely  similar  in  number  and  po- 
sition. 

It  will  therefore  be  obvious,  that  when  the  upper 
millstone  is  set  in  its  place,  the  directions  of  these 
grooves  will  cross  each  other.  The  surface  of  the 
lower  stone  is  convex,  that  of  the  upper  concave, 
and  the  convexity  of  the  former  has  less  height  than 
the  concavity  of  the  latter  has  depth. 

The  grain,  falling  into  the  space  between  the  mill- 
stones, is  caught  by  the  upper  one  in  its  revolution, 
and  being  partly  crushed  and  partly  cut  by  the  pro- 
jections of  the  grooves,  is  carried  outward  by  the  cen- 
trifugal  force,  until  it  passes  between  the  circumfer- 


264 


PRACTICAL    MECHANICS. 


ence  of  the  upper  millstone  and  the  surface  of  the 
lower,  into  a  box  which  surrounds  both,  whence  the 
meal  is  discharged  by  a  spout. 

Fig.  83  represents  a  section  of  a  pair  of  millstones, 
with  their  accessories. 

Fig.  83. 


D.  Upper  millstone. 
R.  Lower  millstone. 

K.  Spindle  which  carries  the  upper  millstone. 
S.  Hopper. 

Q.  Moveable  hopper,  called,  from  its  shape,  the  shoe.   This  is  sha- 
ken from  side  to  side  by  four  bars  fastened  to  the  top  of  the 


spindle. 

1  opi 
the  grain  falls  from  the  shoe. 


k.  Eye  of  the  mill,  an  opening  with  a  short  funnel,  through  which 


M  M.  Case  into  which  the  meal  is  thrown  from  between  the  mill- 
stones by  the  centrifugal  force  derived  from  the  rotary  motion 
of  the  upper  millstone. 

266.  It  has  been  found  that  the  force  of  a  single 
man  is  capable  of  giving  motion  to  an  iron  mill  of 
9  French  inches  in  diameter,  at  the  rate  of  30  revo- 
lutions in  a  minute.  The  product  is  20  metrical 
pounds  per  hour.  A  mill  for  two  men  has  millstones 
21  inches  in  diameter,  and  makes  80  revolutions  per 
minute.     The  product  is  doubled. 

In  order  to  construct  a  mill  for  the  power  of  a 
single  horse  or  of  seven  men,  it  would  be  necessary, 
in  making  it  move  at  the  same  rate,  to  give  it  a  di- 
ameter of  31  inches  :  but  it  is  made  to  revolve  120 


PRACTICAL   MECHANICS.  265 

times  in  a  minute,  and  the  surface  is  diminished  to 
the  diameter  of  32  inches. 

The  French  mills  which  are  moved  by  water  have 
a  diameter  of  72  inches,  and  make  from  70  to  72 
revolutions  in  a  minute.  These  require  the  force  of 
four  horses.  Many  English  mills  have  a  diameter 
of  four  feet,  and  turn  with  a  velocity  of  120  revolu- 
tions in  a  minute.  The  force  of  three  horses  is  suf- 
ficient to  move  them.  The  best  English  mills  have 
a  diameter  of  five  feet,  and  revolve  90  times  in  a 
minute. 

267.  In  the  American  flouring  mills,  the  grain  is 
raised  from  the  vessels  which  carry  it  to  the  mills, 
or  from  a  reservoir  into  which  it  is  thrown  from  the 
sacks,  by  an  apparatus  called  the  elevator.  This  is 
composed  of  a  number  of  small  copper  buckets  at- 
tached to  a  leathern  strap,  and  worked  by  the  force 
which  moves  the  mill,  after  the  manner  of  a  chain- 
pump.  From  the  upper  part  of  the  elevator  it  is 
thrown  by  the  motion  of  the  buckets  into  a  screen, 
by  the  action  of  which  it  is  cleansed  from  the  seeds 
of  other  substances  and  the  smaller  grains  of  the 
wheat  itself.  This  screen  is  a  polygonal  prism  en- 
closed in  fine  wires,  which  lies  in  a  position  slightly 
inclined,  and  is  caused  to  revolve  rapidly.  The  grain 
is  thrown  by  the  centrifugal  force  against  the  wires, 
and,  after  many  revolutions,  reaches  the  lower  end 
of  the  screen,  all  the  smaller  seeds  having  escaped 
through  the  screen.  The  screen  discharges  the 
cleansed  wheat  into  the  granary,  where  it  collects  in 
a  heap  on  the  floor. 

In  the  more  perfect  mills,  the  wheat  is  received 
from  the  elevators  into  a  machine  by  which  it  is 
freed  from  smut.  The  smut  machine  is,  in  external 
figure  and  position,  similar  to  the  screen,  but  it  is 
composed  of  one  cylinder  revolving  within  another. 


266  PRACTICAL   MECHANICS. 

The  two  cylinders  are  furnished  with  teeth  or  beaters, 
by  which  the  smutty  grains  of  the  wheat  are  crushed  ; 
these  are  softer  than  the  others,  and  therefore  yield 
easily.  Their  fragments,  with  the  smut  they  con- 
tain, are  separated  by  the  screen. 

From  the  granary  it  is  carried,  as  wanted,  by  an 
apparatus  called  a  conductor.  This  is  composed  of 
a  screw  working  in  a  trough.  By  this  it  is  conveyed 
to  a  point  whence  it  falls  into  the  hopper,  which  is  a 
pyramidal  funnel.  Beneath  this  is  a  moveable  ap- 
paratus, which  is  touched  by  the  millstone  in  its  rev- 
olutions, and  thus  acted  upon  in  such  manner  that 
the  millstone  receives  exactly  as  much  grain  as  will 
suffice  for  properly  feeding  it. 

The  meal,  after  being  discharged  from  between  the 
.stones,  is  received  in  a  horizontal  trough,  in  which  a 
skeleton  screw,  formed  of  leaves  projected  from  a 
solid  axis,  works ;  by  this  it  is  carried  forward  to  a 
reservoir,  whence  it  is  lifted,  either  by  a  similar 
screw  working  in  an  inclined  position,  or  by  an  ele- 
vator, to  the  upper  story  of  the  mill.  Falling  upon 
the  floor  of  this,  it  is  spread  out  by  an  apparatus 
called  the  hopper-hoy  or  cooler.  This  is  composed 
of  a  sort  of  rake,  consisting  of  two  equal  arms  revolv- 
ing on  their  centre.  The  teeth  of  the  rake  are  in- 
chned,  and  their  number  on  the  two  arms  is  unequal, 
so  that  those  on  the  one  arm  in  revolving  follow  the 
spaces  between  the  teeth  of  the  other  arm.  The  out- 
er tooth  of  one  of  the  arms  catches  the  meal  as  it 
comes  from  the  elevator,  and  drags  it  over  the  floor 
in  the  circumference  of  a  circle.  The  outer  tooth  of 
the  other  arm  moves  the  flour  thus  spread  nearer  to 
the  centre  of  the  apparatus,  and  so  in  succession  un- 
til the  meal  reaches  a  hole  near  the  centre.  Through 
this  hole  it  falls  by  its  own  weight  into  the  bolting 
machine.     The  bolting  machine  has  the  same  shape 


PRACTICAL   MECHANICS.  267 

as  the  screen,  but  is  enclosed  in  a  fine  cloth,  and  the 
passage  of  the  finely-ground  flour  through  the  cloth 
is  aided  by  pieces  of  wood,  called  beaters,  arranged 
at  the  edges  of  a  prism  similar,  but  somewhat  larger 
than  that  which  holds  the  bolting  cloth.  The  latter, 
when  loaded  with  the  meal,  is  sufficiently  flexible  to 
be  thrown  against  the  beaters  by  the  centrifugal  force. 

Within  the  bolting  machine  are  arrangements  by 
which  the  husk  of  the  grain  and  the  coarser  parts 
are  separated  into  portions  of  different  value  and 
fineness. 

From  the  bolting  machine  the  superfine  flour  is 
conveyed  by  machinery  to  a  bin,  in  which  it  remains 
until  it  is  to  be  packed,  when,  by  merely  opening  a 
small  gate  in  a  spout,  it  is  permitted  to  fall  into  the 
barrel.  Into  this  it  is  rammed  by  machinery,  and 
the  workman  has  no  more  to  do  than  to  close  the 
gate  as  soon  as  a  barrel  is  filled,  remove  it,  replace 
it  by  an  empty  one,  and  so  on,  until  the  whole  of 
the  grain  is  ground,  or  the  mill  is  stopped. 

In  addition,  there  are  in  the  best  mills  apparatus 
for  weighing  the  wheat,  and  the  flour  in  the  barrels. 
The  whole  of  the  work,  from  the  unlading  of  the 
vessels  in  which  it  is  brought,  to  the  package  in 
barrels,  is  performed  by  machinery,  and  a  single 
man  with  a  boy  are  sufficient  to  perform  the  work 
of  an  extensive  mill. 

The  distribution  of  the  more  important  of  these 
parts  may  be  understood  from  Fig.  84,  on  page  268, 
in  which  the  interior  of  a  mill  of  four  run  of  stones 
is  represented. 

a  a.   Elevators   by  which   the     e  e.  Hoppers. 

grain  is  raised.  / /.    Conducting    screws    by 

B.  Granary.  which  the  meal  is  conveyed 

C.  Screen.  to  the  elevator  g^  ^g-, 

D  D.  Millstones;  one  of  these,    H.  Cooler  or  Hopper-boy. 
with  its  grooved  face,  is  shown    I.  Bolting  xMachine. 
atK. 


268  PRACTICAL    MECHANICS. 

Fig.  84. 


^  268.  Grist  mills  may  be  moved  by  any  of  the 
kmds  of  water-wheel  which  we  have  described,  or 
by  a  steam-engine.  It  is  estimated  that  for  each 
pair  of  stones,  with  the  labour-saving  machinery, 
the  power  of  four  horses  is  sufficient.     The  hori. 


PRACTICAL    MECHANICS.  269 

zontal  form  of  wheels  has  the  merit  of  admitting  of 
the  utmost  simplicity  in  their  arrangement,  for  the 
spindle  which  carries  the  upper  millstone  may  also 
be  the  axle  of  the  horizontal  wheel.  It  is,  however, 
more  difficult  to  regulate  the  velocity  of  these  with- 
in the  proper  limits,  and  in  many  cases  they  would 
cause  a  great  waste  of  the  power.  It  is  therefore 
better,  in  large  manufacturing  mills,  to  employ  an 
undershot,  overshot,  or  breast  wheel,  according  to  the 
circumstances  of  the  case  ;  and  as  both  the  plane  in 
which  they  move,  and  the  velocity  best  adapted  to 
their  most  efficient  action,  are  different  from  that  of 
the  millstone,  the  proper  changes  in  the  direction 
and  rate  of  the  motions  are  effected  by  means  of 
bevelled  and  spur  wheels.* 

269.  In  mills  driven  by  a  water-wheel,  the  motion 
is  usually  taken  off  at  right  angles,  and  altered  in  ve- 
locity by  a  single  combination,  which  was  in  the  old 
mills  composed  of  a  cog-wheel  and  trundle,  but  which 
is  now  made  up  of  a  bevelled  wheel  and  bevelled  pin- 
ion. Using  an  undershot  wheel  having  a  diameter 
of  15  feet,  which  is  a  good  proportion.  Dr.  Brewster 
gives  the  following  table  for  a  stone  of  five  feet  in 
diameter,  making  90  revolutions  per  minute. 

*  As  an  example  of  such  an  arrangement,  we  shall  take  that 
of  a  mill  moved  by  a  steam-engine  making  30  strokes  per  minute. 
On  the  axle  of  the  crank  is  a  bevelled  wheel  having  84  teeth. 
This  works  into  a  second  bevelled  wheel,  by  which  the  horizon- 
tal motion  is  rendered  vertical,  and  which  has  72  teeth.  The 
third  wheel  is  mounted  upon  the  same  axle  as  the  second,  and  has 
36  teeth.  The  fourth  wheel  has  41  teeth,  and  the  upper  millstone 
is  mounted  on  a  spindle  passing  through  its  centre.  The  relation 
between  the  number  of  strokes  of  the  engine  and  the  number  of 
revolutions  of  the  millstone  will  be  thus  expressed  : 
84xl36_ 

The  millstone  will  therefore  make  116.1  revolutions  per  minute. 


Revolutions 

Relation  between  the 

Teeth  0 

wheel.    1 

of  wheel  per 

revolutions  of  the 

n  the 

minute. 

wheel  and  stones. 

Pimos 

4.16 

21.63 

130 

6 

4.62 

15.31 

92 

6 

7.20 

12.50 

100 

8 

8.32 

10  81 

97 

9 

9.28 

9.70 

97 

10 

8 

8.83 

97 

11 

8.64 

8.19 

90 

11 

270  PRACTICAL    MECHANICS. 

Veloeitf 
of  Water. 

7.62 

10.77 
13.26 
15.24 
17.04 
18.67 
20.15 

The  relations  in  the  overshot  and  breast  wheels, 
whose  circumferences  move  with  a  constant  velocity, 
can  be  calculated  with  great  care.  The  following 
table  is  given  by  Brewster  for  the  use  of  the  conden- 
sing low-pressure  engine  in  driving  grist-mills.* 

4  12.5  28  29.8 

8  16.75  32  32 

12  20.2  36  34.2 

16  23.25  40  36 

20  26.25  44  38 

24  28.1  48  39.5 

*  As  a  farther  illustration  of  this  subject,  we  give  a  description 
of  a  mill  at  Rochester,  N.  V.  The  water-wheels  are  three  in 
number,  each  of  which  drives  three  run  of  stones,  and.  is  18 
feet  in  diameter.  The  buckets  are  11  feet  3  inches  in  length 
in  the  clear.  The  head  of  water  is  18  feet,  and  is  usually  admit- 
ted upon  the  wheel  at  the  distance  of  three  feet  from  its  upright 
diameter.  This  (^  38)  we  have  seen  to  be  a  more  advanta- 
geous mode  than  if  the  wheel  were  made  small  enough  to  let  the 
water  spout  against  its  upper  bucket.  The  gate  through  which 
the  water  passes  is  3^  inches  high,  and  is  usually  drawn  2  inches. 

Each  wheel  has  on  its  axle  a  bevel-wheel  11  feet  in  diameter, 
and  having  144  teeth.  These  wheels  drive  horizontal  pinions 
having  36  teeth,  on  the  same  axle  with  which  are  spur-wheels  10 
feet  in  diameter,  having  144  cogs.  These  drive  pinions  of  25 
teeth,  which  carry  the  millstones. 

The  millstones  are  5  feet  in  diameter,  and  make  140  revolutions 
per  minute,  and  each  run  of  stones  grinds  from  8  to  JO  bushels  of 
wheat  per  hour.  The  usual  product  of  the  mill,  having  nine  run 
of  stones,  is  froro  300  to  400  barrels  of  flour  in  24  hours.  But  in 
a  press  of  work,  as  much  as  600  barrels  have  occasionally  been 
manufactured. 

To  sift  the  flour  there  are  four  large  bolting  chests,  each  con- 
taining four  reels  30  inches  in  diameter  and  18  feet  in  length- 


PRACTICAL    MECHANICS.  271 

Saw-Mills. 

270.  Saw-mills  have  also  constituted  an  impor- 
tant branch  of  American  industry.  These,  as  erect- 
ed in  most  places,  are  of  the  simplest  possible  struc- 
ture. There  is  but  one  saw,  which  is  mounted  in  a 
frame,  to  which  the  appropriate  oscillating  motion 
is  given  by  a  small  wheel,  or  rather  axle  furnished 
with  four  leaves.  On  this  the  water  is  admitted  by 
an  inclined  spout  reaching  to  the  head  of  the  fall. 
On  the  axle  of  this  wheel  is  a  crank  connected  with 
the  frame  by  a  rod.  The  log  to  be  sawn  is  laid  on 
a  frame,  which  is  pushed  forward  at  each  oscillation 
of  the  frame  by  a  ratchet  wheel. 

This  mode  is  only  to  be  praised  for  its  small  cost 
and  simplicity,  but  it  is  a  very  inefficient  method  of 
using  the  water,  being  driven  by  an  undershot  wheel 
of  the  worst  structure.  The  saw-mills  of  greater 
power,  which  carry  gangs  of  saws  in  sufficient 
number  to  cut  a  log  at  one  operation  into  the  great- 
est number  of  boards  into  which  it  can  be  divided, 
are  driven  by  an  overshot  wheel,  the  motion  of  which 

There  are  also  four  hopper-boys  for  cooling  the  meal  before  it  is 
bolted ;  a  bolting  chest  with  a  single  reel  for  middlings,  and  a 
duster  for  rebolting  the  bran.  The  reels  are  42  inches  in  diame- 
ter and  22  feet  long. 

The  elevator  raises  the  grain  from  boats  to  a  height  of  48  feet, 
and  is  of  sufficient  capacity  to  raise  1000  bushels  in  an  hour  and 
a  half;  and  there  are  arrangements  for  weighing  the  grain  as  fast 
as  it  is  elevated. 

According  to  the  ratio  of  the  number  of  teeth  on  the  wheels 
and  pinions,  the  water-wheels  make  about  six  revolutions  per 
minute,  and  the  rate  at  which  their  circumference  moves  is  about 
six  feet  per  second. 

As  the  water  is  apt  to  fail  in  times  of  drought,  a  steam-engine  of 
80  horse  power  is  attached,  which  can  be  geared  to  the  upright 
shafts,  driven  by  two  of  the  wheels,  and  thus  six  run  of  stones  may, 
if  necessary,  be  driven  by  steam,  while  three  are  driven  by  the  re- 
maining water-wheel. 

For  cleaning  the  wheat  there  are  three  screens,  two  smut  ma- 
chines, and  three  fanning- mills. 


272        PRACTICAL  MECHANICS. 

is  changed  in  velocity  by  a  wheel  and  pinion,  to  the 
axle  of  the  latter  of  which  a  crank  is  applied. 

A  saw-mill  of  this  construction  is  represented  in 
Fig.  85. 

Fig.  85. 


A  A.  Overshot  water-wheel,  on  the  circumference  of  which  cogs 

are  placed. 
B.  Pinion  and  crank,  which,  by  means  of  the  connecting  rod  c. 

gives  motion  to  the  frame  d,  which  carries  the  saw. 


PRrACTICAL   MECHANICS.  273 

E.  Ratchet  wheel,  which  derives  its  motion  from  the  frame  d  in 
such  manner  as  to  be  carried  forward  one  tooth  at  each  cut  of 
the  saw,  and  thus  force  up  the  moveable  carriage//,  on  which 
the  log  to  be  sawn  is  placed. 

G.  Pmion  which  can  be  thrown  in  and  out  of  gear  by  the  system 
of  levers  i  i  k.  On  the  axle  of  this  pinion  is  a  band,  which 
gives  motion  to  a  wheel  L,  by  which  the  carriage //is  returned 
to  its  original  position  after  the  log  has  been  sawn  through. 

Self-acting  apparatus,  by  which  the  gate  may  be 
closed,  the  wheel  which  withdraws  the  carriage 
thrown  into  gear,  and  the  log  shifted  the  thickness 
of  the  scantling  between  each  motion  of  the  car- 
riage, have  been  added  -  to  mills ;  the  logs  are  also 
drawn  up  on  an  inclined  plane  by  the  machinery. 

271.  Instead  of  a  straight  saw  oscillating  in  a  frame, 
circular  saws  have  been  recently  much  employed. 
Some  difficulties  have  been  found  in  applying  them 
to  sawing  heavy  timber  ;  but  for  sawing  veneers,  and 
all  light  work,  they  are  to  be  preferred,  in  conse- 
quence of  the  greater  rapidity  with  which  they  per- 
form their  work,  the  greater  smoothness  of  the  cuts 
they  make,  and  a  saving  in  power.  In  cutting  ve- 
neers, the  axle  of  the  wheel  has  in  some  cases  been 
made  to  rise  and  fall,  thus  combining  the  advantages 
of  the  oscillating  and  the  circular  motion. 

One  of  the  most  complete  and  important  combi- 
nations of  circular  saws,  is  that  employed  in  Glas- 
gow (Scotland)  in  the  fabrication  of  barrels. 

272.  Planing  machines  may  be  considered  with 
saw-mills.  The  work  of  planing  wood  is  effected  in 
these  by  knives  placed  in  an  oblique  direction  on 
the  circumference  of  a  cylinder.  A  machine  for 
tonguing  and  grooving  plank  for  flooring  has  been 
constructed  on  the  same  principles.  The  grooves 
are  cut  by  a  thin  cylinder,  having  cutting  teeth  on 
its  circumference.     The  tongue  is  formed  by  two 


274  PRACTICAL    MECHANICS. 

such  cylinders  revolving  parallel  to  each  other  on 
the  same  axle.  The  planks  are  reduced  to  the 
proper  width  by  passing  them  between  two  circular 
saws.  These  circular  saws,  the  planing  knives,  and 
the  tonguing  and  grooving  instruments,  are  so  com- 
bined that  the  whole  process  is  performed  at  one 
operation,  and  a  rough  plank  introduced  at  one  end 
of  the  machine,  comes  out  finished  and  fit  for  laying 
in  a  floor  at  the  other.         -v 

Cotton  Spinning. 

273.  Of  all  American  products  cotton  is  now  the 
most  important,  from  its  value  as  an  export,  and  the 
great  extent  to  which  its  domestic  consumption  has 
been  carried.  This  article  is  of  two  different  spe- 
cies, the  long  and  the  short  staple.  The  former  is 
easily  cleansed  of  its  seed  by  a  simple  apparatus 
composed  of  toothed  rollers.  The  short  staple  cot- 
ton is  cleansed  with  greater  difficulty,  but  the  pro- 
cess is  now  effectually  performed  by  an  ingenious 
instrument,  invented  by  \Vhitney,  and  called  the  saw- 
gin.  The  essential  part  of  this  apparatus  is  an  axle, 
B,  to  which  are  adapted  a  number  of  fine  circular 
saws.  A  section  of  this  axle  and  one  of  the  saws  is 
represented  at  A,  in  Fig.  86.  The  bolls  of  cotton, 
with  the  seed  attached  to  it,  being  thrown  into  the 
hopper  D,  the  points  of  the  saws,  at  h,  in  their  revo- 
lution, tear  the  cotton  from  the  seeds,  and  the  latter, 
being  left  free,  drop  through  an  aperture  in  the  bot- 
tom of  the  hopper,  which  is  regulated  by  a  screw  so 
as  just  to  permit  them  to  pass.  The  cotton,  carried 
forward  by  the  teeth  of  the  saws,  is  removed  from 
them  by  brushes,  E  E,  which  revolve  on  the  circum- 
ference of  the  wheel  F  F  ;  it  is  thus  thrown  upon 
the  inclined  plane  C,  whence  it  descends  to  a  recep- 
tacle beneath  the  machine.     Nothing  can  well  be 


PRACTICAL    MECHANICS.  275 

Fig.  86. 


imagined  more  simple  than  this  engine  ;  yet,  simple 
as  it  is,  it  has  created  the  most  important  source  of 
our  national  wealth.  The  services  of  Whitney  to 
the  United  States  are  hardly  less  valuable  than  those 
of  Watt  to  England ;  and  it  is  a  matter  of  national 
reproach,  that  these  services  were  unrewarded,  ex- 
cept by  a  grant  from  the  State  of  South  Carolina, 
and  that  far  from  bearing  any  relation  to  the  im- 
mense wealth  created  by  the  machine. 

274.  The  cotton,  after  being  cleansed,  is  packed  in 
bales,  which  are  usually  compressed,  and  the  first 
operation  in  the  manufacture  consists  in  picking  and 
opening  it,  for  the  purpose  of  separating  any  re- 
maining seeds  or  other  matter,  and  fitting  it  to  be 
taken  up  by  the  next  machine  in  the  order  of  the 
process. 

This  operation  was  for  a  long  time  wholly  per- 
formed by  hand.  It  is  now  aided  by  machines,  which 
go  under  the  name  of  willows  or  winnows.  The 
cotton  thus  picked  and  opened  is  placed  upon  a 
cloth,  whose  ends  are  united  in  such  manner  as  to 
form  an  endless  band,  represented  at  A,  which 
passes  over  two  rollers,  and  carries  the  cotton  for- 
ward to  feed  the  blowing  machine.     This  machine 


276         PRACTICAL  MECHANICS. 
Fig.  87. 


has  two  rollers  with  grooved  surfaces,  which  draw 

the  cotton  between  them,  and  expose  it  to  the  action 

L  J        ]  ^''^'^  ^^^""^"^  ^''  ^^^te^'s  B,  which  make  8 

or  900  revolutions  in  a  minute.     These  beaters,  af 


PRACTICAL  MECHANICS.  277 

ter  having  partially  opened  the  cotton,  throw  it  for- 
ward upon  an  incUned  grating,  whence  it  passes 
to  a  second  band  C,  revolving  on  rollers,  which  car- 
ries it  forward  to  a  second  pair  of  grooved  roll- 
ers. These  draw  it  in  and  deliver  it  to  a  second 
jfly  with  beaters  D,  by  which  it  is  farther  opened  and 
thrown  upon  a  second  grating.  During  this  process 
the  cotton  is  exposed  to  a  current  of  air  produced 
by  a  revolving  fan,  by  which  the  dust  and  impuri- 
ties it  contains  are  driven  through  the  gratings,  or 
up  the  chimney  G.  Finally,  the  cotton  is  carried 
forward  from  the  gratings  upon  a  third  band  E, 
until  it  passes  out  upon  a  frame  or  grating  F, 
where  it  is  collected  in  order  to  feed  a  second  ma- 
chine of  similar  character,  called  the  batting  or  lap- 
ping machine.  By  this  it  is  not  only  cleansed  of 
any  remaining  impurities,  but  caused  to  form  itself 
into  a  species  of  web  called  a  bat.  In  order  to 
form  the  bat,  the  cotton  is  delivered  by  the  second 
band  of  cloth  to  a  cylinder  formed  of  wire  gauze. 
The  air  is  drawn  from  the  inside  of  this  cyhnder  by 
a  revolving  fan,  placed  in  a  flue  provided  for  carry- 
ing oflT  the  dust ;  and  the  external  air  passing  to- 
wards  the  cyUnder  in  order  to  supply  that  displaced, 
presses  the  cotton  against  its  surface,  whence  it  is 
drawn  off  by  a  solid  cylinder,  and  subjected,  as  it 
rolls  itself  upon  it,  to  pressure  from  another  c^'^inder. 
275.  The  coXon  bats  are  next  carded.  Before 
machinery  was  introduced,  cotton  was  carded  by 
hand.  The  cards  used  for  this  purpose  were  com- 
posed of  wire,  adapted  to  a  sheet  of  leather,  and  fur- 
nished with  a  handle.  Cards  of  similar  character 
are  now  placed  upon  the  surface  of  a  cylinder.  In 
order  to  their  proper  adaptation  to  this  purpose,  it  is 
necessary  that  they  be  constructed  with  the  greatest 
accuracy.  The  regularity  and  evenness  of  the  thread 
22 


278  PRACTICAL    MECHANICS. 

is  in  a  great  measure  due  to  the  regularity  and  per- 
fection of  the  operation  of  carding.  It  would  be  im- 
possible, without  the  aid  of  machinery,  to  give  to  the 
cards  that  degree  of  accuracy  which  is  now  aimed 
at,  A  machine  has,  in  consequence,  been  invented  by 
Whittemore,  which  manufactures  cards  with  absolute 
accuracy.  The  leather  is  split  by  it  with  extreme 
precision  into  thin  layers  ;  the  holes  to  receive  the 
teeth  are  pierced  in  it  with  the  utmost  regularity  ; 
the  wire  is  cut  of  exactly  equal  lengths,  bent  into  a 
form  comprising  four  angles,  to  which  identical  meas- 
ures are  given,  and  implanted  in  the  leather. 

The  carding  is  performed  at  two  operations.  In 
the  first,  the  bat  is  laid  upon  a  band  of  cloth,  by  which 
it  is  carried  to  two  fluted  rollers  ;  these  present  it  to 
a  large  cylinder,  covered  on  the  outside  with  cards. 
Over  the  upper  part  of  this  is  a  portion  of  a  hollow 
cylinder,  lined  with  cards,  whose  teeth  are  bent  in  a 
direction  opposite  to  those  of  the  former.  Between 
these  the  fibres  of  the  cotton  are  drawn,  and  arranged 
in  parallel  directions.  The  cotton  is  gradually  car- 
ried forward  until  it  reaches  a  cylinder  called  the 
doflTer,  which  has  cards  upon  it,  arranged  in  the  form 
of  a  spiral.  The  cotton  is  drawn  off  by  the  teeth  of 
the  cards  on  the  doffer,  and  thence  is  removed  by  a 
comb,  ^  which  an  oscillating  motion  in  the  vertical 
direction  is  given  by  a  crank.  By  this  arrangement 
of  doffer  and  comb,  a  continuous  band  or  lap  is  form- 
ed, which  is  wound  upon  a  cylinder,  and  at  the  same 
time  compressed  by  a  roller. 

The  lap  is  cut  off"  from  time  to  time,  and  presented 
to  the  feeding  rollers  of  the  finishing  cards.  These 
have  the  same  structure  as  the  first,  but  the  roll  which 
is  combed  of  the  doffer,  instead  of  being  wound  upon 
a  cylinder  and  compressed,  is  received  into  a  tin  can. 

The  structure  of  the  carding  machine  is  represent- 
ed in  fig.  88,  on  page  278. 


PRACTICAL   MECHANICS. 


279 


The  cotton  in  bats  is  introduced  at  A  upon  a  cloth, 
the  ends  of  which  are  sewed  together,  and  which  is 
passed  over  two  rollers.  These  rollers  being  set  in 
motion  by  the  engine,  the  cloth  revolves  and  carries 
the  cotton  forward  to  two  rollers  at  B,  by  which  it  is 
drawn  within  reach  of  the  cards.  These  are  ar- 
ranged upon  the  surface  of  a  large  drum  or  cylinder 
C.  Over  this  cylinder  is  a  portion  of  a  hollow  cyl- 
Fig.  88. 


280  PRACTICAL    MECHANICS. 

inder  D,  the  inside  of  which  is  also  covered  with 
cards.  After  passing  between  the  drum  and  the 
portion  of  the  hollow  cylinder,  the  cotton  is  caught 
upon  a  third  set  of  cards,  arranged  upon  a  cylinder 
in  the  form  of  a  screw.  From  this  cylinder  the  cot- 
ton is  removed  in  a  continuous  roll,  by  the  comb  F, 
and  is  wound  upon  the  cylinder  or  doffer.  The 
wheels,  band,  and  crank  by  which  motion  is  given  to 
the  comb,  will  be  seen,  and  require  no  reference. 

276.  The  materials,  which  are  composed  of  fila- 
ments of  such  a  character  as  to  be  capable  of  forming 
threads,  were  originally  spun  by  hand,  aided  only  by 
the  rude  apparatus  called  the  distaff  and  spindle. 
Machines  have,  however,  long  been  used  to  facilitate 
the  operation,  under  the  name  of  spinning-wheels. 
Of  these  there  are  two  kinds,  the  great  and  the  small 
wheel.  These  have  served  as  the  basis  of  the  two 
principal  engines,  by  which  the  force  of  the  great  nat- 
ural agents  has  been  substituted  for  human  labour  in 
spinning. 

277.  The  great  wheel  is  set  in  motion  by  the  hand 
of  the  spinner,  who,  giving  to  it  several  impulses  in 
rapid  succession,  causes  a  rotary  motion,  which  the 
wheel  retains  for  a  time,  in  consequence  of  its  iner- 
tia. By  the  revolution  of  the  wheel,  motion  is  given 
to  a  spindle.  The  material  having  been  previously 
formed  by  cards  into  rolls,  the  end  of  one  of  these  is 
rolled  until  it  will  be  held  upon  the  spindle,  and,  after 
the  latter  has  received  its  motion,  the  spinner,  holding 
the  roll  between  the  fingers,  draws  it  in  a  direction 
oblique  to  the  plane  in  which  the  wheel  revolves. 
In  this  way  the  roll  is  drawn  out,  stretched,  and,  at 
the  same  time,  twisted.  Having  thus  drawn  out  a 
thread,  the  end  of  it  is  shifted  until  it  comes  into  a 
plane  parallel  to  that  in  which  the  wheel  revolves. 


PRACTICAL    MECHANICS.  281 

When  in  this  position,  the  revolution  of  the  spindle 
coils  the  thread  upon  it.  New  rolls  are  attached 
by  pressure  to  the  unspun  end  of  the  former  one,  and 
the  operation  is  repeated  until  as  large  a  continuous 
thread  as  the  spindle  will  receive,  is  wound  upon  it. 
This  machine  was  principally  employed  in  spinning 
wool  of  short  staple,  and  is  also  suited  to  cotton, 
which  may,  however,  be  spun  in  the  other  manner. 

278.  The  small  wheel  is  set  in  motion  by  a 
treadle,  or  step  furnished  with  a  hinge,  to  which  an 
oscillating  motion  is  given  by  the  foot.  The  wheel 
has  a  crank  on  its  axle,  which  is  attached  to  the 
treadle  by  a  connecting  rod,  and  thus,  like  the  fly- 
wheel of  the  steam-engine,  acquires  a  continuous 
rotary  motion.  A  rapid  motion  is  given  to  a  spin- 
dle by  means  of  a  band  passing  over  the  circumfer- 
ence of  the  wheel.  The  spindle  of  this  wheel  is  not 
a  simple  rod  of  iron  of  the  appropriate  form,  but  has 
upon  its  end  a  part  called  the  fly.  This  has  the 
form  represented  by  A  in  Fig.  89. 

Upon  the  spindle  is  placed  a  bobbin,  composed  of 
two  disks  united  by  a  hollow  cylinder.  The  hollow 
is  of  such  size  as  to  slip  readily  over  the  spindle^ 
and  does  not  revolve  with  it,  unless  a  force  of  inten' 
sity  nearly  sufficient  to  break  the  thread  should  as* 
sist  the  revolution.  Upon  one  side  of  the  fly  are 
several  short  wires  bent  into  the  form  of  hooks 
The  matter  to  be  spun  being  plac^  upon  a  distaflT, 
which  is  mounted  on  the  machine  within  reach  ot 
the  spinner,  a  small  portion  is  twisted  by  the  fingers 
into  the  form  of  thread,  drawn  through  an  eye  on 
the  top  of  the  spindle,  thence  through  one  of  the 
hooks,  and  fastened  to  the  bobbin.  The  wheel  be- 
ing set  in  motion,  the  fingers  are  still  applied  to 
unite  and  twist  the  filaments  as  they  are  drawn  from 
the  distaff*  by  the  revolution  of  the  spindle,  and  the 


282 


PRACTICAL    MECHANICS. 


thread  is  moved  in  succession  from  one  hook  to  the 
next,  in  order  that  it  may  be  wound  uniformly  upon 
the  bobbin.  By  this  instrument,  prepared  flax, 
combed  wool,  and  cotton,  the  latter  previously  spun 
on  the  great  wheel,  may  be  formed  into  threads, 
which  are  more  closely  twisted  than  can  be  done  by 
that  instrument. 

An    improved   spinning-wheel  is  represented    in 


Fig.  89. 


Fig.  89.  In  the  upper 
part  of  the  instrument 
is  seen  the  distaff,  on 
which  the  prepared  ma- 
terial is  wound ;  next 
below  are  the  bobbin 
and  fly ;  in  the  middle 
space,  to  the  left,  is  the 
wheel  over  which  the 
band  that  gives  motion 
to  the  fly  is  passed ; 
beneath  all  is  the  trea- 
dle, whence  a  rod  pro- 
ceeds to  the  crank  on 
the  axle  of  the  wheel. 
The  improvement  con- 
sists in  an  endless  screw 
on  the  axle  of  the  wheel, 
which  gives  motion  to 
a  toothed  wheel.  On 
the  same  axle  with  the 
toothed  wheel  is  a  heart 
wheel  or  eccentric  ;  this 
presses  against  a  bar,  which  is  pressed  on  the  oppo- 
site side  by  a  spring.  The  upper  end  of  the  bar  is 
adjusted  to  the  axle  of  the  fly,  which  is  caused  by  it 
to  move  to  and  fro,  and  thus  to  distribute  the  thread 
uniformly  on  the  bobbin. 


PRACTICAL  MECHANICS.         283 

279.  It  will  be  obvious,  tbat  in  the  first  of  these 
instruments,  the  only  part  of  the  labour  which  re- 
quired strength  was  the  continual  walking  to  and 
from  the  wheel ;  while  in  the  latter  no  appreciable 
exertion  of  force  was  required.  Hence,  although 
very  superior  to  the  rude  apparatus  of  distaff  and 
spindle,  they  were  very  unprofitable  applications  of 
human  strength.  The  great  difficulty,  either  in  com- 
bining such  a  number  of  spindles  as  might  require 
the  exertion  of  the  whole  strength,  or  driving  them 
by  some  natural  agent,  which  should  render  human 
labour  unnecessary,  consists  in  finding  a  substitute 
for  the  nice  mechanism  of  the  human  hand  in  the 
small  wheel,  or  to  endow  a  machine  with  the  intel- 
ligence by  which,  at  an  appropriate  time  in  the  mo- 
tion of  the  great  wheel,  the  act  of  twisting  and 
drawing  should  be  succeeded  by  one  of  mere  stretch- 
ing upon  a  thread  already  formed.  It  might  almost 
seem  that  such  qualities  in  machines  could  not  be 
given  except  by  a  creative  power,  and  yet  they  have 
been  attained  in  the  inventions  of  Arkwright,  Har- 
graves,  and  Compton.  Even  these  great  inventors 
did  little  more  towards  the  present  state  of  the  art, 
than  to  exhibit  the  principles  in  successful  opera- 
tion. Almost  every  day  witnesses  improvements  in 
the  plan  of  the  subsidiary  machinery,  and  the  in- 
creasing perfection  of  the  essential  parts.  In  these 
improvements  American  ingenuity  has  borne  its  full 
share,  and  it  has  recently  been  stated  by  good  au- 
thority,* that  the  most  important  machines  used  in 
this  manufacture,  for  which  patents  are  still  in  force 
in  Great  Britain,  are  the  property  of  American  in- 
ventors. 

By  the  force  of  this  ingenuity,  the  manufacture  of 

*  Speech  of  Mr.  Villiers  in  the  House  of  Coramons,  February, 
1839. 


284  PRACTICAL    MECHANICS. 

cotton  in  this  country,  which,  little  more  than  20 
years  since,  was  utterly  prostrated  before  the  supe- 
rior cheapness  of  that  of  Great  Britain,  has  been 
increased  until  our  goods  not  only  compete  with 
those  of  that  country,  in  some  cases  to  their  actual 
exclusion,  in  markets  where  both  are  liable  to  equal 
iiscal  charges,  but  actually  rival  them  in  the  shops 
of  Calcutta,  where  the  British  articles  have  the  ben- 
efit of  a  protecting  duty. 

The  quantity  of  cotton  manufactured  in  the  Uni- 
ted States  is  now  as  great  as  was  consumed  in  Great 
Britain  in  1814 ;  the  Southern  planters  have  found 
a  new  market  equal  to  a  fourth  of  their  whole  crop, 
and  the  Noi'thern  wheat-growers  receive  a  price  for 
their  product  not  graduated  by  the  cost  of  produc- 
tion, but  by  that  of  importation  from  foreign  coun- 
tries. However  foreign  to  our  subject,  we  cannot 
here  help  remarking,  that  in  the  face  of  these  facts, 
the  agriculturists  of  the  United  States  are  taught 
by  political  theorists  to  look  with  jealousy  on  the 
prosperity  of  their  manufacturing  brethren. 

280.  It  would  exceed  our  limits  to  enter  into  the 
detail  of  the  processes  in  their  present  improved 
state.  It  is  sufficient  for  our  purpose  to  explain 
the  principles  on  which  the  more  important  ma* 
chines  are  founded. 

The  cotton  having  been  carded  into  bands  or 
slivers,  which  are  received,  as  has  been  stated,  in 
cans,  is  subjected  to  a  process  called  drawing.  By 
this  several  bands  are  united  into  one  loosely  spun 
thread,  and  have  their  length  considerably  increased. 
For  this  purpose,  the  ends  of  the  several  bands  are 
each  passed  between  two  pairs  of  rollers,  a  a  and  h  h. 
The  lower  roller  of  each  pair  is  moved  by  machin- 
ery, the  upper  is  set  in  motion  by  the  friction  which 
the  band  undergoes  in  passing  it.     The  lower  roll- 


PRACTICAL    MECHANICS. 


285 


ers  are  made  of  iron  and  fluted,  the  upper  roller  is 
covered  with  leather.  The  second  pair  of  rollers 
has  a  greater  velocity  than  the  first,  and  the  band  of 
cotton  is  thus  increased  in  length.  After  passing 
the  second  pair  of  rollers,  several  of  the  bands  are 
drawn  together  by  passing  them  through  a  conical 
ring  c,  and  are  thence  drawn  in  a  body  through  a 
third  pair  of  rollers,  whence  they  are  delivered  into 
a  tin  can. 

Fig.  90 


286 


PRACTICAL    MECHANICS. 


281.  The  process  of  roving,  which  follows  next, 
is  also  perfornaed  by  rollers,  and  in  a  similar  manner, 
but  the  rovings  are  delivered  into  cans  which  have 
a  revolving  motion.  This  motion  answers  the  dou- 
ble purpose  of  coiling  the  rovings  neatly  in  the  can, 
and  giving  them  a  slight  twist. 

The  machine  on  which  this  is  performed  is  rep- 
resented in  Fig.  91. 

Fig.  91. 


The  slivers  are  drawn  from  the  cans  into  which 
they  were  delivered  by  the  drawing  frame,  by  the 
rollers  A.  Thence  they  pass  to  the  rollers  B, 
where   thev   are   doubled.      The   doubled   strands 


PRACTICAL    MECHANICS.  287 

then  drop  into  the  revolving  cans  C  C,  which  are 
kept  in  motion  by  wheels  and  bands. 

The  operation  of  drawing  is  often  repeated  several 
times,  in  which  case  the  rovings,  instead  of  being  re- 
ceived in  revolving  cans,  are  wound  upon  bobbins  by 
means  of  a  spindle  and  fly,  as  we  shall  describe  under 
the  head  of  throstle  spinning.  The  only  difference  is, 
that  in  this  case  the  apparatus  is  less  delicate. 

Instead  of  roving,  this  is  sometimes  called  coarse 
bobbin  and  fly  spinning, 

282.  At  this  stage  the  manufacture  diverges  into 
two  different  parts,  known  as  mule  and  throstle  spin- 
ning. By  the  former,  a  loosely-spun  thread,  used  as 
the  filling  or  woof  in  weaving,  is  obtained  ;  by  the 
second,  one  strong  and  tightly  spun,  known  under 
the  name  of  twist,  and  employed  as  warp  or  as  sew- 
ing-thread. 

283.  Mule  spinning  is  performed  in  two  separate 
steps,  on  machines  known  as  the  mule  and  the  stretch- 
ing frame.  In  the  latter,  the  bobbins  or  cans  which 
have  received  the  rovings  are  placed  in  a  fixed  frame, 
and  drawn  between  three  successive  pairs  of  rollers ; 
thence  they  pass  to  a  spindle  situated  upon  a  movea- 
ble frame  or  carriage,  whose  wheels  run  upon  a 
railway.  At  a  particular  stage  of  the  motion,  the 
drawing  rollers  cease  to  act,  and  the  farther  retreat 
of  the  carriage  tends  to  stretch  or  extend  the  length 
of  the  roving  which  has  already  been  delivered.  At 
the  same  time,  a  greater  degree  of  closeness  is  given  to 
the  thread,  by  making  the  spindles  of  the  moveable 
frame  revolve  more  rapidly,  from  which  act  the  ma- 
chine is  called  a  double  speeder.  The  carriage, 
having  receded  to  the  extent  of  the  railroad,  is  return- 
ed towards  the  frame  which  contains  the  rollers,  and 
at  the  same  instant  the  position  of  the  spindles  is 


288  PRACTICAL    MECHANICS. 

changed,  so  that  the  thread  may  be  wound  upon 
them. 

284.  The  mule  differs  from  the  stretching  frame 
only  in  the  fact,  that  the  threads  are  delivered  to  the 
spindles  during  the  whole  receding  motion  of  the  car- 
riage or  mule,  and  that  the  double  speed  is  omitted. 
The  mule  is  fed  from  rovings,  the  stretching  frame 
from  the  spindles  which  have  been  filled  by  the  mule. 

285.  The  motion  of  the  carriages  which  bear  the 
spindles  was  at  first  wholly  performed  by  the  labour 
of  men.  It  is  now,  however,  so  far  aided  by  the 
machinery,  that  the  mule  or  stretching  frame  only  re- 
quires to  be  returned  by  their  exertion.  The  frames 
have,  therefore,  been  gradually  increased  in  size,  un- 
til, from  containing  no  more  than  one  hundred  and 
fifty  spindles,  they  have  been  made  in  fine  work  to 
comprise  eleven  hundred.  The  operation  of  mule 
spinning  is  the  only  one  which  requires  the  strength 
of  a  full-grown  man.  All  the  other  operations  in 
cotton-spinning  are  superintended,  rather  than  per- 
formed, by  women  and  children.  Each  mule-spin- 
ner can  manage  two  frames,  which  act  alternately ; 
but  he  requires  the  assistance  of  two  or  more  per- 
sons,  according  to  the  number  of  spindles  in  the 
frames.  The  most  important  duty  of  these  is  to 
watch  and  piece  the  broken  threads. 

286.  In  throstle  spinning  the  rovings  are  drawn 
out  by  successive  pairs  of  rollers,  in  which,  by  in- 
creasing the  velocity  as  before,  they  are  extended  in 
length.  They  are  then  wound  upon  a  bobbin  by 
means  of  a  spindle  and  fly,  imitated  from  the  small 
wheel.     The  apparatus  is  represented  in  Fig.  92. 

The  thread  is  drawn  from  the  bobbin  a,  through  a 
series  of  rollers  at  h.  It  is  thence  carried  through 
an  opening  in  a  plate,  and  bent  downward  until  it  is 


PRACTICAL    MECHANICS. 

Fig.  92. 


289 


passed  through  an  eye  in  the  arm  of  the  fly  c,  whence 
it  proceeds  to  the  bobbin  d.  In  order  to  wind  the 
thread  uniformly  on  the  latter,  the  rail  e,  on  which 
^he  bobbin  rests,  is  caused  to  move  alternately  up 
and  down.  It  will  be  obvious  that  the  winding  of 
the  thread  on  the  bobbin  will  only  take  place  in  con- 
sequence  of  the  difference  between  its  velocity  and 
that  of  the  spindle.  ,  n    x.    i.     a 

Motion  is  given  to  the  spindle  and  fly  by  bands. 


290 


PRACTICAL    xMECHAxXICS. 

Fig.  93. 


PRACTICAL    MECHANICS.  291 

and  to  the  rollers  by  toothed  wheels,  which  will  be 
seen  in  the  figure. 

In  later  forms  of  the  machine,  one  of  the  arms  of 
the  fly  is  made  hollow,  and  the  thread  is  passed 
through  it. 

In  a  recent  throstle  spinning  machine,  the  fly  has 
assumed  the  form  of  a  hollow  cylinder,  and  this  is 
considered  to  be  an  important  improvement. 

287.  According  to  the  degree  of  fineness  required 
for  the  threads,  the  operation  of  drawing  is  repeated 
two,  three,  four,  or  even  as  many  as  eight  times. 
Even  in  coarse  yarns,  three  successive  drawings  are 
often  performed.  In  the  first,  six  slivers  are  united 
into  one  ;  in  the  second,  six  of  the  first ;  and  in  the 
third,  nine  of  those  formed  by  the  second  drawing. 
The  degree  of  extension  of  the  rolls  from  the  time 
they  leave  the  cards  is  therefore 

6x6x9=324. 

When  six  successive  drawings  are  employed,  the 
extension  may  be  represented  by 

8X4X7X6X6X6=42,384. 

In  the  finest  spinning  it  is  sometimes  as  much  as 
70,000.  These  degrees  of  fineness  can  only  be  ob- 
tained from  Sea  Island  cotton. 

288.  The  motion  of  the  mules  in  the  finer  spin- 
ning is  slower  than  in  the  coarser.  Thus,  in  spin* 
ning  thread  denominated  No.  40,  from  that  number 
of  hanks,  each  of  840  yards,  being  contained  in  a 
pound,  the  mule  makes  three  motions  in  a  minute, 
while  in  the  higher  numbers  no  more  than  one  is 
performed  in  the  same  space  of  time. 

289.  The  latest  improvement  which  has  taken 
place  in  this  art  is  the  introduction  of  the  self-acting 
mule.     The  spinners  of  Great  Britain  having,  under 


292  PRACTICAL   MECHANICS. 

false  views  of  their  interest,  associated  in  trades 
unions,  the  owners  of  the  manufacturing  establish- 
ments were  compelled  to  seek  for  such  improve- 
ments in  machinery  as  might  render  them  independ- 
ent of  their  workmen,  and  were  successful.  This 
machine  is  one  of  the  most  splendid  triumphs  of  hu- 
man ingenuity  ;  and,  so  far  from  having  interfered 
with  the  comforts  of  the  spinners,  it  seems  to  have 
rather  advanced  their  prosperity,  by  enabling  them 
to  free  themselves  from  a  thraldom  imposed  by  the 
idle  and  dissipated  members  of  their  own  body. 

It  happens,  fortunately,  that  all  improvements  can 
be  but  slowly  introduced,  ^s  they  demand  the  sacri- 
fice of  the  capital  already  invested  in  the  older 
forms.  Thus,  while  the  combination  was  broken 
up,  the  means  of  occupation  still  remained  in  the 
old  machinery. 

The  self-acting  mule  has  been  recently  executed 
successfully  in  the  United  States,  and  will  probably 
aid  most  materially  in  the  extension  of  the  cotton 
manufacture  among  us. 

The  common  mule  is  represented  in  Fig.  93,  on 
page  290. 

290.  The  business  of  cotton  spinning  has  been 
much  decried  as  injurious  to  the  health  and  morals 
of  the  persons  employed  in  it.  This,  however,  has 
been  thoroughly  disproved  by  accurate  statistics  in 
Europe,  whereby  it  appears  that  the  factory  work- 
men are  not  only  better  paid,  but  have  a  better 
chance  of  life  and  higher  moral  character  than  the 
agricultural  labourers.  In  that  country  it  has  cer- 
tainly tended  to  elevate  a  debased  population,  and, 
therefore,  it  is  not  to  be  feared  that  in  ours  it  will 
lower  the  standard  of  morals,  while  it  will  add  ma- 
terially to  the  comfort  and  independence  of  those 
who  must  be  supported  by  the  labour  of  their  hands. 


PRACTICAL   MECHANICS.  293 

There  can  be  no  comparison  between  the  comforts 
enjoyed  by  the  females  employed  in  manufacturing 
establishments,  and  those  who  accompany  their  fam- 
iUes  in  seeking  agricultural  establishments  in  the 
new  states  of  the  West. 

Flax  Spinning, 

291.  Cotton  adapts  itself  to  manufacture  by  ma- 
chinery with  greater  ease  than  any  other  of  the  ma- 
terials whence  threads  can  be  spun.  But,  in  spite  of 
the  difficulties  which  have  opposed  the  spinning  of 
wool,  worsted,  and  flax  by  machinery,  they  have 
been  overcome  in  a  great  degree.  The  general  or- 
der and  character  of  the  methods  is  similar  to  that 
employed  in  the  spinning  of  cotton,  and  the  ma- 
chinery used  in  the  latter  has  served  as  the  type  of 
that  which  has  succeeded  in  the  other  cases.  Each 
different  material,  however,  requires  modifications  in 
the  structure  and  mode  of  working  the  apparatus, 
which,  if  they  involve  no  important  difference  of 
principle,  are,  notwithstanding,  absolutely  essential 
to  the  success  of  the  operation.  The  process  by 
which  coarse  thread  is  spun  from  flax  may  be  now 
considered  to  be  nearly  as  complete  as  that  used 
in  spinning  cotton ;  but  the  completion  of  this  pro- 
cess  does  not  promise  to  lead  to  results  in  any  de- 
gree as  valuable  as  those  which  have  attended  the 
cotton  manufacture.  The  difference  is  owing  to 
the  great  diversity  in  the  preparation  of  the  two 
substances.  Cotton  is  fit  for  carding  when  it  leaves 
the  gin  of  Whitney,  and  is  only  subjected  to  inter- 
mediate processes  in  consequence  of  the  compres- 
sion it  has  undergone  in  packing,  and  the  dirt  which 
has  entered  it.  These  intermediate  processes  are 
simple  and  easy.  Flax,  on  the  other  hand,  requires 
a  long  and  troublesome  process  to  separate  the  fibre 


294  PRACTICAL    MECHANICS. 

from  the  matter  with  which  it  is  combined,  arid 
must  undergo  several  successive  operations  of  great- 
er difficulty  than  carding,  before  it  can  be  spun. 

292.  The  subsequent  processes  are  drawing,  ro- 
ving, and  spinning,  as  in  the  manufacture  of  cotton ; 
but  it  is  necessary  that  the  flax  be  kept  wet,  which 
renders  the  manufacture  disagreeable  to  the  persons 
employed. 

292.  Flax  has,  until  recently,  been  obtained  from 
the  plant  by  the  process  of  rotting  or  retting.  In 
this  the  bark  and  woody  fibre  of  the  plant  are  de- 
composed. This  operation  may  be  performed  by 
exposure  to  air  and  moisture  upon  a  meadow.  It  is 
then  called  dew-retting,  but  is  more  effectually  ac- 
complished by  steeping  the  flax  in  ponds  or  artificial 
canals  for  10  or  12  days.  The  water  with  which 
these  are  charged  ought  to  be  soft.  Exposure  on 
the  grass  for  a  few  days  is  sufficient  to  complete  the 
process. 

This  method  is  tedious  and  unwholesome,  not  only 
to  those  employed  in  it,  but  to  those  who  reside  in 
the  neighbourhood.  The  flax  also  is  liable  to  injury 
from  the  putrefaction  being  carried  to  too  great  an 
extent,  while  it  may  be,  if  the  decomposition  is  not 
sufficiently  advanced,  but  partially  freed  from  the 
woody  fibre.  Various  attempts  have,  in  conse- 
quence, been  made  to  destroy  the  impurities  of  the 
plant  by  machinery.  None  of  these  seem  to  have 
been  successful  as  a  preparation  for  the  finer  fabrics. 

The  decomposed  woody  matter  is  separated  by  a 
process  called  breaking.  This  was  formerly  per- 
formed entirely  by  hand,  but  is  now  effected  by  ma- 
chinery. Breaking  is  performed  by  blunt  iron  teeth, 
fixed  in  pieces  of  wood,  one  set  of  whicl^  is  fixed  and 
the  other  «ioveable.    When  performed  by  machinery, 


PRACTICAL    MECHANICS.  295 

these  may  be  arranged  on  cylinders,  like  the  cards 
in  the  cotton  carding  machine.  The  separation  is 
completed  by  scutching,  which  consists  in  beating  the 
flax  against  a  post,  and  with  an  instrument  resem- 
bling a  curry-comb. 

The  material,  after  being  scutched,  passes  into  the 
hands  of  the  manufacturer.  The  first  and  prelimi- 
nary part  of  the  manufacture  is  called  hackling. 
The  hackle  is  composed  of  many  teeth  firmly  fasten- 
ed in  a  board.  It  appears  probable  that  this  opera- 
tion might  be  accomplished  by  machinery. 

Spinning  of  Woollen  and  Worsted, 

294.  The  first  operation  which  wool  undergoes,  as 
a  preparation  for  spinning  by  machinery,  is  perform- 
ed in  a  mill  called  a  devil.  This  is  composed  of  a 
cylinder  about  two  and  a  half  feet  in  diameter,  and 
five  rollers  which  lie  above  the  cylinder.  The  cyl- 
inder and  rollers  are  covered  with  teeth,  which  catch 
into  each  other,  those  of  the  cylinder  interlocking 
with  those  of  all  the  rollers,  and  the  teeth  of  the  sev- 
eral rollers  interlocking  with  each  other.  The  wool 
is  charged  through  a  door,  in  a  case  which  covers 
the  instrument,  and  is  discharged  by  the  centrifugal 
motion  of  the  cylinder,  at  another  door  provided  for 
the  purpose. 

295.  The  second  operation  is  called  scribbling, 
and  is  performed  by  cards  placed  upon  cylinders,  like 
those  used  in  the  cotton  batting  machine.  Instead 
of  a  fixed  concave  cylinder  covered  with  cards,  as  in 
that  machine,  the  revolving  cylinder  is  surrounded 
by  pairs  of  small  cylinders  also  covered  with  cards. 
The  teeth  of  the  opposite  cards  do  not,  as  in  the  case 
of  cotton,  intersect  each  other,  but  are  merely  brought 
so  near  that  a  few  fibres  of  the  wool  held  by  the 
teeth  of  one  card  can  be  caught  and  drawn  out  by 


296  PRACTICAL    MECHANICS. 

the  teeth  of  the  other.  The  wool  being  collected  on 
a  doffer,  is  stripped  from  it  by  a  comb  in-  a  continu- 
ous fleece.  The  operation  of  scribbling  is  repeated 
three  times. 

These  fleeces  are  then  passed  through  the  carding 
machine,  which  is  constructed  on  the  same  principle 
as  the  cotton  carding  machine ;  but  the  dofler,  instead 
of  being  entirely  covered  with  teeth,  is  studded  with 
strips  of  card  about  four  inches  in  breadth.  The 
wool  is  therefore  stripped  ofl*  in  fine  webs  of  that  di- 
mension. These  fall  into  a  trough  of  the  shape  of 
a  half  cylinder,  and  are  formed  into  rolls  by  a  fluted 
cylinder  which  revolves  within  the  trough, 

296.  The  rolls  are  next  passed  through  a  machine, 
called  the  slubbing  billy.  This  is  analogous  to  the 
roving  machine  of  the  cotton  manufacture,  but  has 
rarely  been  successfully  driven  by  any  other  prime 
mover  except  human  strength.  The  subsequent  pro- 
cesses are  so  like  those  employed  in  the  cotton  man- 
ufacture, that  they  do  not  require  description,  and, 
with  the  exception  of  the  scribbling  and  carding  ma- 
chines, whose  differences  can  be  understood  by  a 
verbal  description,  the  distinction  between  the  ma- 
chines used  in  spinning  cotton  and  wool  are  so  slight, 
that  the  same  figures  will  serve  to  explain  both 
This  has,  however,  regard  only  to  the  principle  ;  for 
there  is  so  great  a  distinction  in  the  characters  of 
the  two  materials,  that  a  machine  which  has  been 
constructed  for  spinning  the  one  cannot  be  employed 
in  the  other. 

297.  Worsted  is  a  variety  of  the  woollen  manu- 
facture, in  which  a  wool  of  long  staple  or  fibre  is 
used,  and  is  treated  in  such  a  way  that  all  the  fibres 
shall  be  laid  parallel.  The  stuff  prepared  from  it, 
therefore,  can  have  no  nap  or  pile  raised  upon  it. 


PRACTICAL    MECHANICS.  297 

Instead  of  carding,  by  which  the  longer  fibres  are 
broken  up,  an  operation  called  combing  is  employed, 
in  which  the  longer  fibres  are  laid  parallel,  and  the 
shorter  fibres  separated. 

The  comb  is  composed  of  two  rows  of  pointed 
teeth  made  of  tempered  steel.  Two  combs  are  ne- 
cessary, and  they  have  handles  attached  when  the 
process  is  to  be  performed  by  men.  One  of  the  combs 
is  fixed  with  the  teeth  uppermost,  on  which  the  wool 
is  placed,  and  thence  drawn  oflT  upon  the  other  comb. 
The  combs  require  to  be  kept  warm,  as  there  is  a 
particular  temperature  at  which  the  process  can  be 
most  conveniently  and  successfully  conducted. 

Combing  by  machinery  has  been  found  a  difficult 
process,  but  has,  at  length,  been  successfully  accom- 
plished. The  combs  are  placed  on  two  wheels  which 
revolve  in  opposite  directions,  so  that  the  teeth  of  the 
opposite  combs  shall  meet  each  other  in  their  re- 
spective revolutions.  One  of  the  cylinders  revolves 
on  a  fixed  axis,  the  axle  of  the  other  has  a  recipro- 
cating motion,  by  which  it  approaches  and  recedes 
from  the  fixed  cylinder  four  times  in  each  revolution. 

The  machine  is  made  to  register  its  own  motions, 
and  rings  a  bell  when  it  ought  to  be  stopped,  in  order 
to  remove  the  combs  which  have  become  charged 
with  wool. 

These  combs  are  then  heated,  and  placed  in  an- 
other machine  resembling  in  action  the  doffer  of  a 
cotton  mill,  by  which  the  wool  is  drawn  off*  in  a  con- 
tinuous roll  or  sliver.  This  is  received  in  a  tin  cyl- 
inder. 

298.  Hand-combed  wool  is  formed  by  the  hand 
into  rolls  of  from  five  to  seven  feet  in  length.  In 
applying  these  to  the  next  machine,  it  is  necessary 
to  lay  them  on  an  inclined  plane  of  plank,  and  to  join 


298  PRACTICAL   MECHANICS. 

the  ends  by  hand ;  while  wool  combed  by  the  ma- 
chine is  drawn  out  from  the  cans. 

299.  The  next  process  resembles  in  character  the 
drawing  and  doubling  of  the  cotton  manufacture,  and 
is  performed  on  an  apparatus  called  the  breaking 
machine.  The  thread  is  formed  by  successive  oper- 
ations of  drawing,  spinning,  and  twisting,  by  methods 
and  machines  which  have  a  general  resemblance  to 
those  used  in  the  cotton  manufacture. 

Silk  Manufacture. 

300.  The  production  of  silk  promises  to  be  an  ob- 
ject of  great  importance  in  the  United  States.  Our 
climate  has  a  remarkable  resemblance  to  that  of 
China,  the  native  country  of  the  silkworm ;  and  not 
only  do  the  European  and  Asiatic  varieties  of  the 
mulberry,  which  is  the  proper  food  of  the  silkworm, 
flourish  from  Connecticut,  or  even  farther  north,  to 
Florida,  but  we  have  a  native  species,  which,  from 
late  experiments,  appears  to  be  as  well  adapted  to  the 
purpose  as  any  of  the  foreign. 

301.  The  silkworm  spins  a  web  of  the  form  of 
an  egg,  called  a  cocoon,  in  which  it  encloses  itself. 
The  insect  must  be  killed  by  the  heat  of  the  sun  or 
of  an  oven,  or  by  the  steam  of  boiling  water.  In  this 
state  the  cocoon  may  be  kept  for  a  short  time,  but 
cannot  be  transported  to  any  great  distance.  In  or- 
der to  convert  the  filaments  of  the  cocoon  into  the 
raw  silk,  in  which  form  it  becomes  an  article  of  com- 
merce, they  must  be  drawn  from  the  cocoons  and 
formed  into  skeins.  This  operation  is  thus  perform, 
ed  :  the  cocoons  are  thrown  into  a  vessel  of  water, 
which  is  placed  upon  a  charcoal  fire,  in  order  to 
maintain  the  liquid  at  an  elevated  temperature  ;  the 


PRACTICAL    MECHANICS.  299 

warm  water  dissolves  the  gummy  animal  matter  by 
which  the  thread  of  the  cocoon  is  held  together  ;  a 
whisk  or  broom  being  then  plunged  into  the  water, 
such  filaments  as  it  may  come  in  contact  with  ad- 
here to  it,  and  may  be  drawn  out.  A  number,  gen- 
erally four,  of  these  are  taken,  twisted  together  by 
the  fingers,  passed  through  an  eye  made  of  wire,  and 
attached  to  the  reel ;  this  operation  is  repeated,  and 
four  more  filaments  twisted  together  are  applied  to 
another  part  of  the  reel,  in  such  a  manner  that  two 
threads  may  be  reeled  at  a  time  ;  the  reel  is  then 
turned  by  hand,  requiring  no  greater  force  than  the 
strength  of  a  child,  and  a  grown  person  attends  to 
attach  new  filaments,  as  those  of  the  first  cocoon 
are  expended. 

The  reel  is  so  constructed  as  to  have  an  alterna- 
ting motion  along  its  axis,  through  a  space  of  about 
three  inches,  in  order  that  the  successive  parts  of 
the  thread  may  not  fall  upon  each  other,  in  which 
case  they  might  adhere  by  the  gum  which  coats  their 
surface.  But  a  breadth  of  three  inches  is  sufficient 
to  allow  the  gum  to  become  dry. 

The  reels  are  also  moved  by  machinery,  in  estab- 
lishments called  filatures. 

302.  Raw  silk  thus  formed  into  skeins  is  an  im- 
portant article  of  commerce,  for  in  many  of  the  coun- 
tries which  produce  silk  it  is  not  manufactured  far- 
ther. It  would  appear  probable  that  it  would  for  a 
long  time  be  more  profitable  in  this  country  to  pur- 
sue this  plan,  for  the  labour  required  in  the  successive 
operations  by  which  it  is  fitted  for  weaving  is  so 
great,  in  spite  of  the  introduction  of  spinning  machin- 
ery, that  it  can  be  performed  in  countries  where  hu- 
man strength  is  less  in  demand  on  more  advantage- 
ous terms  than  in  the  United  States. 

More  profit  will  be  derived  from  the  preparation 


300  PRACTICAL   MECHANICS. 

of  raw  silk  from  the  greater  part  of  the  cocoons, 
than  in  that  of  sewing-silk,  to  which  it  has  princi- 
pally been  applied  in  the  United  States. 

303.  The  first  step  in  the  manufacture  of  silk  from 
its  raw  state  is  called  winding.  In  it  the  skeins  are 
replaced  upon  reels,  and  drawn  thence  upon  bobbins. 
Winding  was  formerly  performed  by  hand,  on  a  ma- 
chine carrying  four  bobbins,  drawing  from  a  like 
number  of  reels.  Similar  machines  are  now  driven 
by  water  or  a  steam-engine.  The  great  difficulty 
in  this  operation  consists  in  the  necessity  of  mending 
the  threads  which  break,  and  in  some  of  the  silk-mills 
of  England  no  fewer  than  one  thousand  children  are 
employed  in  this  operation  alone. 

304.  The  subsequent  processes  are  doubling  and 
throwing,  or  twisting.  In  the  former,  two,  three,  or 
more  threads  are  laid  together  on  the  same  bobbin. 
In  the  latter,  these  are  wound  upon  another  bobbin 
by  means  of  a  spindle  and  fly,  as  in  the  throstle  spin- 
ning of  the  cotton  manufacture. 

Weaving  and  Finishing, 

305.  Weaving  is  performed  upon  an  engine  called 
the  loom,  which  is  among  the  most  ancient  of  all  ma- 
chines. Woven  cloth  is  composed  of  two  sets  of 
threads  crossing  each  other  at  right  angles.  Those 
which  extend  lengthwise  are  called  the  warp,  the 
cross  threads  are  called  the  iboof  or  filling.  When 
the  filling  passes  up  and  down  between  all  the  threads 
of  the  warp,  the  cloth  has  a  uniform  surface.  By 
making  it  pass  above  or  under  more  than  one  thread, 
before  it  changes  to  the  opposite  face  of  the  cloth,  a 
twill  or  diagonal  edge  may  be  raised,  or  patterns  of 
any  figure  may  be  formed,  as  in  the  article  known  by 
the  name  of  damask. 

306.  In  order  to  prepare  the  warp  for  the  loom, 


PRACTICAL   MECHANICS. 


301 


each  thread  of  which  it  is  made  up  must  be  wound 
in  a  separate  roll  upon  a  beam.  This  was  formerly 
a  very  laborious  operation,  but  is  now  performed  by 
a  very  ingenious  machine  of  American  invention. 

The  manner  in  which  weaving  is  performed  will 
be  best  understood  from  Fig.  94,  which  represents  a 
loom  of  the  most  ancient  and  still  most  usual  form. 
Fig.  94. 


24 


302  PRACTICAL    MECHANICS. 

A  A  A  A.  Wooden  frame  which  encloses  the  loom. 

B.  Beam  on  which  the  warp  is  wound. 

aa,b  b.  Heddies,  These  are  two  in  number,  and  are  each  formed 
of  two  rods,  a  and  6,  united  by  threads.  The  threads  are  looped 
near  the  middle,  and  the  threads  of  the  warp  are  passed  through 

'  these  loops.  The  first  thread  is  passed  through  the  first  loop 
on  a,  the  second  through  the  first  loop  on  6,  and  thus  alternately 
until  all  the  warp  has  been  used. 

The  heddies  are  suspended  from  pullies  by  cords  reaching  to  the 
treadles  D  and  E.  By  applying  the  foot  to  the  treadles,  one  of 
the  heddies  is  raised  and  the  other  is  depressed.  The  alternate 
threads  of  the  loop  are  thus  separated,  leaving  an  opening  be- 
tween them  for  the  passage  of  the  shuttle. 

H  H,  Batten.  This  is  a  frame  of  wood  which  swings  on  hinges 
from  the  upper  part  of  the  frame  ;  the  latter  is  furnished  below 
with  a  frame,  which  receives  the  reed.  The  threads  of  the 
warp  are  passed  through  the  reed  after  they  leave  the  heddies, 
and  proceed  thence  to  the  cloth  beam  K. 

The  lower  bar  of  the  batten  projects  on  each  side  beyond  the  ver- 
tical bars  H  H,  and  extends  about  an  inch  and  a  half  in  front  of 
them,  forming  a  rail  or  shelf  on  which  the  shuttle  runs. 

L.  Seat.       "^^ 

The  shuttle  is  represented  on  a  larger  scale  at  S. 
It  is  an  oblong  piece  of  wood,  pointed  at  both  ends,  and 
furnished  with  two  rollers,  to  cause  it  to  run  on  its 
race  with  little  friction.  The  ends  of  the  shuttle-race 
are  r^cnedi  into  troughs,  1 1,  by  means  of  thin  boards, 
and  in  each  of  these  is  a  piece  of  horn  or  thick  leath- 
er, called  a  pecker.  The  peckers  have  holes  in  them, 
through  each  of  which  a  short  wire  is  passed  to  serve 
as  a  guide.  Each  pecker  has  a  string  fastened  to  it, 
and  the  two  strings  meet  in  a  handle,  which  is  held 
by  the  weaver.  The  shuttle  being  placed  in  the  race 
between  the  peckers,  the  weaver  jerks  them  alter- 
nately by  means  of  the  strings,  and  the  two  peckers 
are  thus  caused  to  strike  the  shuttle  in  succession, 
and  throw  it  to  and  fro  between  the  threads  of  the 
warp.  Between  each  motion  of  the  shuttle,  the 
weaver,  by  applying  his  feet  alternately  to  the  treadles, 
causes  the  heddies  to  rise  and  fall,  and  thus  closes 
the  warp  behind  the  thread  which  is  carried  by  the 
shuttle.  The  batten  is  then  drawn  up  forcibly,  and 
completes  the  web, 


PRACTICAL   MECHANICS.  303 

307.  The  thread  which  is  carried  by  the  shuttle 
is  wound  on  a  small  bobbin,  which  is  fitted  on  a  wire 
or  spindle  lying  in  a  rectangular  mortise  formed  in 
the  upper  surface  of  the  shuttle. 

308.  In  order  to  form  a  tweel  or  other  pattern, 
more  than  one  pair  of  heddles  is  used  ;  and  these 
are  connected  with  treadles,  which  set  them  in  mo- 
tion when  they  are  needed. 

The  heddles  are  sometimes  so  numerous  as  to  re- 
quire the  aid  of  a  boy  to  work  them. 

309.  Before  the  addition  of  the  peckers  to  the 
loom  and  the  application  of  rollers  to  the  shuttle, 
the  latter  was  thrown  from  the  hand  of  the  weaver, 
provided  the  cloth  were  narrow ;  but  in  wide 
webs  it  was  necessary  for  an  assistant  to  be  placed 
on  each  side  of  the  loom,  to  recover  and  return  the 
shuttle. 

310.  The  motions  even  of  the  common  loom  are 
so  complex,  that  it  would  appear  difficult  to  substi- 
tute any  other  power  for  that  of  man.  Looms  to 
be  driven  by  water  or  steam  have,  however,  been  in- 
troduced with  success,  under  the  name  of  the  power 
loom.  A  front  view  of  one  of  these  is  represented 
in  Fig.  95,  and  a  side  view  in  Fig.  96,  on  page  304- 

311.  In  order  to  finish  cotton  cloths,  the  pile  or 
loose  fibres  are  first  singed  off.  This  is  performed 
by  drawing  the  fabric  slowly  and  steadily,  by  rollers 
moved  by  water  or  steam,  over  a  cylinder  kept  at 
red  heat. 

The  surface  is  then  rendered  smooth  by  calender- 
ing, which  is  an  operation  similar  to  that  of  man- 
gling. A  box  containing  heavy  weights  may  be 
rolled  over  the  cloth  until  it  is  perfectly  smooth. 
Some  fabrics  are  glazed  by  friction,  after  having 
coated  the  surface  with  a  little  wax. 


304 


PRACTICAL    MECHANICS. 

Fig.  95 


Fig  96. 


The  best  calenders  are  composed  of  rollers,  and 
these  have  been  slightly  modified  to  answer  the  pur- 
pose of  glazing  also.     When  they  have  this  form 


PRACTICAL  MECHANICS.         305 

the  cloth  is  passed  through  them  with  a  continuous 
motion. 

312.  The  finishing  of  woollen  cloths  begins  with 
the  process  of  fulling.  This  consists  in  agitating 
and  exposing  new  surfaces  to  the  action  of  water 
charged  with  fuller's  earth  or  soap. 

This  operation  is  performed  by  mallets  hung  from 
a  fixed  axle,  on  which  they  have  a  reciprocating 
motion.  The  faces  of  the  mallets  are  cut  into  steps, 
so  as  to  double  and  bend  the  cloth  in  various  di- 
rections. 

313.  After  the  fulling  is  completed,  the  cloth  is 
stretched  on  vertical  frames  between  tenter-hooks, 
in  order  that  it  may  not  shrink  too  much  in  drying. 
While  in  this  position,  any  knots  or  uneven  parts 
are  removed  from  the  surface  of  the  cloth,  and  any 
holes  which  exist  are  darned  up.  The  cloth  is  then 
beaten  again  for  some  hours  in  the  fulling  mill,  for 
the  purpose  of  causing  the  thread  to  adhere  in  the 
manner  of  felt.  This  operation  is  repeated  several 
times,  and  the  cloth  thus  becomes  capable  of  receiv- 
ing a  fine  surface,  and  is  less  pervious  to  water. 

314.  The  next  process  is  called  dressing.  This 
consists  in  raising  the  nap  of  the  cloth.  The  only 
article  which  has  been  found  to  answer  this  purpose 
is  the  boll  of  a  species  of  thistle  called  a  teazle,  A 
number  of  these  were  form.erly  arranged  on  frames, 
like  those  of  the  cards  originally  employed  in  the 
cotton  manufacture.  They  are  now  arranged  on  a 
cylinder  which  is  driven  by  machinery. 

315.  The  nap  thus  raised  is  finally  shorn.  This 
was  formerly  done  by  hand,  using  large  shears 
whose  blades  were  fastened  together  by  a  steel 
spring.  Various  attempts  have  been  made  to  shear 
cloth  by  machinery,  and  some  of  these  have  been 


306         PRACTICAL  MECHANICS. 

successful.  In  the  most  elegant  of  these  methods 
the  nap  is  cut  by  spiral  knives  arranged  on  the 
surface  of  a  round  rod  or  small  cylinder,  and  the 
cloth  is  drawn  forward  by  rollers. 

Printing  Machines. 

316.  The  printing  press  in  its  original  form  was 
extremely  rude,  being  no  more  than  a  common  screw 
press  with  a  set  of  ways,  on  which  the  form  contain- 
ing the  type  could  be  pushed  beneath  the  point  of 
pressure,  and  withdrawn  between  each  motion  of  the 
'platen,  by  which  the  paper  was  forcibly  urged 
against  the  type.  The  great  defect  of  this  instru- 
ment,  namely,  that  it  required  as  many  revolutions 
of  the  screw  to  raise  the  platen  as  had  been  em- 
ployed in  bringing  it  down,  was  remedied  by  a 
Dutch  artist  of  the  name  of  Blaew,  who  added  a 
spring  by  which  the  platen  was  lifted  the  moment 
the  action  of  the  lever,  by  which  the  screw  was 
moved,  ceased.  In  this  state  the  printing  press  con- 
tinued until  the  beginning  of  the  present  century. 
From  that  time  the  alterations  and  improvements 
which  have  been  introduced  have  been  too  numer- 
ous to  allow  us  even  to  give  a  list.  The  most  im- 
portant of  all  these  improvements,  without  which  the 
application  of  any  prime  mover  except  the  strength 
of  man  might  perhaps  have  been  impracticable,  is  in 
the  mode  of  inking  the  types.  This  was  formerly 
done  by  means  of  hollow  balls  of  leather.  It  is 
now  universally  performed  by  means  of  a  roller 
composed  of  a  mixture  of  molasses  and  glue,  which, 
after  various  trials,  has  been  found  better  suited  to 
the  purpose  than  any  other  composition  which  has 
been  tried. 

317.  The  most  perfect  printing  machine  which 


PRACTICAL    MECHANICS. 


307 


has  yet  been  constructed  is  that  by  Applegarth  and 
Cowper,  It  is  planned  to  print  both  sides  of  the 
sheet  before  it  leaves  the  machine,  and  turns  out 
from  800  to  1000  sheets  per  hour,  which  is  eight 
times  as  many  as  can  be  performed  by  the  common 
press. 

For  newspaper  printing,  where  one  side  of  the 
sheet  contains  advertisements  and  other  matter 
which  may  be  printed  in  advance  of  the  side  which 
contains  news,  it  is  important  to  print  as  many 
copies  as  possible  of  one  side  of  the  sheet  within  a 
given  time.  Applegarth  and  Cowper  have  con- 
structed one  adapted  to  this  object  for  the  London 
"  Times,"  and  by  means  of  it  4200  impressions 
have  been  obtained  within  the  hour. 

Printing  machines  are  necessarily  complicated. 
We  cannot,  therefore,  undertake  to  describe  one  in 
all  its  details,  but  the  general  principles  on  which 
Applegarth  and  Cowper's  double  press  acts  will  be 
understood  from  Fig.  97. 

B  is  the  feeder,  which  is  composed  of  an  end- 
Fig.  97. 


less  band   of    linen  cloth  stretched  over   two  roll- 
ers  C  and  D.     F  and  G  are  the   printing  cylin- 


308  PRACTICAL    MECHANICS. 

ders,  made  of  iron,  truly  turned,  and  covered  with 
a  fine  woollen  cloth.  Over  these  and  the  subsidi- 
ary cylinders  E,  H,  and  I,  are  stretched  two  sys- 
tems of  endless  tapes,  one  of  which  is  represent- 
ed by  a  continuous,  the  other  by  a  dotted  line. 
Their  direction  and  tension  are  maintained  by  a 
number  of  small  rollers,  whose  sections  appear  in  the 
figure.  The  tapes  are  so  arranged  in  number  and 
distance  from  each  other  as  to  fall  upon  the  parts 
of  the  sheets  which  are  to  be  left  blank. 

Immediately  beneath  this  part  of  the  apparatus  is 
a  long  table,  on  which  are  placed  the  two  forms  of 
type,  and  which  has  an  inking  apparatus  at  each 
end.  On  the  perfection  of  the  latter  the  success  of 
the  process  mainly  depends.  The  table  has  an  al- 
ternating motion,  by  which  the  forms  are  brought 
under  the  printing  cylinders  to  meet  the  sheets,  and 
withdrawn  between  the  times  of  their  passage  under 
those  cyUnders.  The  feeder  B  has  also  a  motion  to 
and  fro,  by  which  the  time  at  which  the  sheets  suc- 
ceed each  other  is  regulated,  and  the  motions  of  the 
table  and  of  the  rollers  are  adjusted  in  exact  con- 
formity. • 

The  paper  being  laid  upon  the  feeder  as  it  re- 
tires, is  carried  in  its  return  within  reach  of  the 
point  at  which  the  two  systems  of  tapes  first  meet 
upon  the  cylinder  E,  and  is  drawn  between  them. 
The  paper  is  thence  carried  in  the  direction  pointed 
out  by  the  arrows,  under  the  printing  cylinder  F, 
where  it  meets  the  first  form  of  type,  and  receives  an 
impression  on  one  side.  It  is  next  carried  over  the 
cylinder  H,  under  the  cylinder  I,  to  the  printing  cyl- 
inder G.  To  this  the  side  which  has  already  been 
printed  by  passing  under  F  is  now  applied  ;  the 
blank  side  of  the  sheet  is  therefore  downward  as  it 
passes  under  the  printing  cylinder  G,  where  it  meets 


PRACTICAL   MECHANICS.  309 

vhe  second  form  of  type  on  the  return  of  the  table. 
After  thus  receiving  an  impression,  the  sheet  is 
thrown  out  printed  on  both  sides  at  K,  where  the  two 
systems  of  types  separate. 

The  machine  may  be  set  in  motion  by  a  band  from 
the  axle  of  the  fly-wheel  of  a  steam-engine.  It  re- 
quires so  little  force  to  turn  it,  that  ten  have  been 
driven  by  an  engine  of  five  horse  power.  It  may 
also  be  turned  by  hand.  When  propelled  by  steam 
it  requires  but  two  persons  to  attend  it,  one  to  lay 
the  paper  on  the  feeder,  the  other  to  receive  the 
printed  sheets. 


310  PRACTICAL   MECHANICS. 


XII. 

ON   MINING. 


318.  Mines  are  excavations  made  in  the  crust  of 
the  earth  for  the  purpose  of  obtaining  useful  minerals. 
From  this  definition  are  to  be  excepted  quarries  of 
stone,  and  works  intended  for  obtaining  clay,  sand, 
and  other  substances  of  little  value.  The  term  is, 
in  fact,  confined  almost  wholly  to  excavations  intend- 
ed to  obtain  the  useful  metals  and  coal. 

319.  The  minerals  which  are  the  objects  of  mi- 
ning, may  be  found :  in  beds  in  alluvial  and  diluvial 
soil,  or  mixed  with  the  sand  and  gravel  of  these  for- 
mations ;  in  regular  strata  in  secondary  formations  ; 
and  in  veins  traversing  the  more  ancient  rocks, 
whether  stratified  or  not.  The  substances  of  value 
which  are  obtained  in  the  first  of  these  positions  are 
the  bog  and  meadow  ores  of  iron,  tin,  and  gold. 
Coal  and  ironstone  are  the  most  important  minerals 
found  in  the  second  class  of  formations.  All  the 
useful  metals  are  found  in  the  third  of  these  posi- 
tions, but  coal  occurs  only  in  the  second. 

320.  Veins  are  fissures  in  rocks,  extending  gen- 
erally to  unknown  depths,  and  which  are  filled  up 
with  minerals  totally  different  from  the  rocks  which 
they  traverse.  These  minerals  are  sometimes  whol- 
ly  earthy,  and  the  veins  are,  in  consequence,  barren. 
When  they  contain  metallic  ores,  earthy  matter  may 
still  form  the  greater  part  of  the  mass  of  the  vein ; 
in  other  cases,  the  metallic  matter  may  be  the  body 
of  the  vein,  and  what  earthy  matter  is  present  may 
be  adventitious.    In  almost  all  cases  veins  are  inclu- 


PRACTICAL  MECHANICS.         311 

ded  in  rocky  envelopes,  differing  from  the  formation 
they  traverse.  So  much  of  this  as  lies  above  the 
vein  is  called  the  roof,  that  which  is  beneath  is  call- 
ed the  floor. 

Veins  are  of  three  descriptions,  the  flat,  the  pipe, 
and  the  rake  vein.  All  of  these  are  subject  to  con- 
traction and  enlargement,  and  the  metallic  matter 
they  contain  may  vary  materially  in  quantity  in  dif- 
ferent parts  of  them.  The  richer  parts,  however, 
usually  occur  in  strings  or  continuous  masses,  gen- 
erally nearly  parallel  to  each  other,  and  having  a 
constant  direction  and  inclination.  These  appear  to 
be  properly  the  lodes,  which  most  writers  have  con- 
founded with  the  veins  themselves.  It  is  by  this  va- 
riety in  thickness,  and  in  being  made  up  of  lodes,  the 
intervals  between  which  may  be  barren,  that  the  flat 
vein  is  to  be  distinguished  from  a  bed  or  regular 
layer  in  a  stratified  formation.  Flat  veins  have 
been  in  many  instances  traced  to  rake  veins,  and 
they  appear  to  be,  in  fact,  branches  of  such  veins, 
which  have  spread  themselves  out  horizontally,  or 
nearly  so,  in  the  joints  and  weaker  parts  of  the  rock 
traversed  by  the  rake  vein.  It  may  however  hap- 
pen, that  convulsions,  which  have  occurred  subsequent 
to  the  formation  of  the  vein,  have  separated  the  flat 
from  the  rake  vein,  whence  it  derived  its  origin. 

Pipe  veins  are  oblong,  rounded  masses,  subject  to 
the  same  law  of  contraction  and  enlargement  as  the 
rake  and  flat  veins.  They  have  probably  been  form- 
ed by  the  branches  of  a  rake  vein,  and  in  the  same 
manner  as  those  which  are  flat,  and  have  in  some  in- 
stances been  traced  to  such  a  source. 

321.  To  discover  mines  is  attended  with  consider- 
able difficulty,  for  geology  as  yet  furnishes  us  with 
no  certain  rules  for  pointing  out  the  positions  in 
which   they  probably  exist.      On   the   other  hand, 


312  PRACTICAL    MECHANICS. 

geology  gives  positive  and  unequivocal  indications, 
whence  it  will  be  at  once  known  that  mines  cannot 
be  found  in  given  sites.  But  even  in  formations  in 
v/hich  mines  may  possibly  exist,  the  fact  of  their 
doing  so  can  only  be  ascertained  by  actual  research. 
Indications  may  sometimes  be  detected  which  may 
encourage  the  undertaking  of  such  researches. 
These  are  fragments  of  ore  in  the  diluvial  soil,  the 
decomposed  rock,  or  the  vegetable  mould.  Great 
care  must  be  taken  to  distinguish  these  from  pieces 
which  have  been  transported  from  a  distance  by  the 
action  of  water,  for  the  source  of  the  latter  may  lie 
at  the  distance  of  miles. 

Minerals  which  occur  in  veins  are  never  found  in 
formations  more  recent  than  the  older  secondary ; 
the  formations  which  are  next  subsequent  in  date  to 
the  coal  measures  are  the  newest  which  are  trav- 
ersed by  metalliferous  veins,  and  it  even  appears 
that  they  do  not  exist  in  them  when  coal  actually  lies. 
beneath.  In  some  coal-fields,  as  in  that  of  Durham-, 
a  few  metalliferous  veins  have  been  found  ;  but  these 
are  rather  an  exception  to  a  general  rule,  and  a  coal 
formation  is  one  of  the  least  likely  places  to  search 
for  the  more  rare  metals.  It  is  otherwise  with  iron ; 
this,  in  a  form  called  iron-stone,  which  is  an  argilla- 
ceous carbonate  of  that  metal,  exists  in  many  coal- 
fields, in  layers  coextensive  with  the  coal  itself.  It 
is,  therefore,  an  inquiry  which  is  well  worthy  of  at- 
tention, whether  this  ore,  so  valuable  from  its  associ- 
ation with  the  fuel  by  which  it  is  best  smelted,  cannot 
be  found.  Metallic  veins  may  exist  in  all  the  forma- 
tions of  more  ancient  date  than  the  coal  measures, 
and  are  more  likely  to  exist  in  a  given  part  of  them 
when  it  has  been  much  disturbed,  and  the  surface  of 
the  ground  is  rugged  and  mountainous.  In  the  older 
stratified    formations,   magnetic   iron,  and   pyrites, 


PRACTICAL   MECHANICS.  313 

often  rich  in  gold,  occur  not  only  in  veins,  but  as  reg- 
ularly stratified  rocks  of  the  formation.  A  full 
knowledge  of  these  rules,  in  a  more  extended  and 
precise  form,  will  enable  the  geologist  to  determine 
whether  a  formation  whence  an  ore  is  brought  is 
such  as  to  render  the  existence  of  a  mine  probable, 
or  its  working  likely  to  be  successful. 

322.  In  the  neighbourhood  of  mines  which  have 
been  worked,  the  search  for  new  bodies  of  mineral 
may  be  conducted  with  much  less  difficulty  than  in 
an  unexplored  region.  Coal,  and  other  minerals 
which  occur  in  strata,  have  often,  for  considerable 
distances,  the  same  inclination ;  and,  even  when  there 
is  a  dislocation  in  the  strata,  these  minerals  still  re- 
tain their  relative  position  to  the  other  rocks  of  the 
formation.  The  lodes  of  flat  veins,  and  the  mass  of 
pipe  veins,  also  maintain  a  direction  and  inclination 
which,  for  any  considerable  distance,  is  constant. 
The  rake  vein,  if  less  regular  than  either  when  view- 
ed within  a  limited  space,  is,  notwithstanding,  even 
more  constant  in  its  general  direction  and  mean  in- 
clination. In  consequence,  when  a  vein  has  been 
opened  at  the  surface,  and  worked  until  its  general  in- 
clination or  dip  become  known,  it  may  be  struck  with- 
in a  few  feet  of  the  calculated  position  by  a  vertical 
pit,  commenced  at  the  distance  of  many  feet  from 
the  place  where  the  vein  crops  out.  So  also  veins 
have  been  traced  in  Cornwall  for  several  miles,  by 
following  the  same  bearing  of  the  compass,  and  ex- 
cavations have  not  failed  to  strike  them  when  not 
the  slightest  indication  was  visible  at  the  surface. 
Coal  may  be  found  with  even  greater  certainty ;  for 
the  extent  and  limits  of  a  coal-field  can  be  ascertain- 
ed with  great  precision,  and  anywhere  within  the 
space  coal  will  be  found,  at  a  depth  which  may  be 


314  PRACTICAL    MECHANICS. 

calculated  from  the  inclination  observed  in  other  parts 
of  the  basin. 

323.  The  place  where  a  body  of  mineral  matter 
appears  at  the  surface,  or  even  when  it  reaches  the 
outer  face  of  a  rock  covered  with  soil  or  disintegra- 
ted rock,  is  called  its  outcrop. 

Where  the  outcrop  of  a  vein  or  stratum  is  expected 
to  exist,  the  search  for  it  may  be  conducted  by  an 
open  trench.  This  is  intended  for  the  purpose  of 
removing  the  alluvial  and  diluvial  deposites,  with 
such  parts  of  the  rock  formation  as  have  been  disin- 
tegrated by  the  action  of  the  weather.  The  direction 
of  such  a  trench  must  be  across  the  probable  direction 
of  the  vein  or  other  mineral  site.  Nothing  more 
can  be  done  in  this  way  than  to  determine  whether 
a  mineral  exist  or  not  in  the  position  where  its  out- 
crop is  suspected  to  be. 

324.  Whether  the  knowledge  of  the  existence  of 
a  vein  or  stratum  be  gained  in  this  or  in  any  other 
way,  its  probable  value,  and  the  mode  in  which  it 
may  be  best  approached  and  entered,  can  only  be 
determined  by  subterranean  works,  unless  the  ne- 
cessary facts  can  be  inferred  from  mines  worked  in 
the  neighbourhood. 

With  these  works  in  the  vein  or  bed  itself,  accu- 
rate geometric  surveys,  both  of  the  horizontal  dimen- 
sions and  level,  must  be  combined,  and  all  the  sub- 
terranean works  must  also  be  carefully  measured,  as 
well  in  their  lineal  dimensions  and  direction,  as  in 
their  inclination  and  change  of  level.  Records  and 
maps  of  such  surveys  must  be  carefully  pres^'ved, 
for  it  is  only  by  reference  to  them  that  any  true 
knowledge  of  the  state  of  the  works  can  be  obtained, 
or  any  sure  plans  laid  down  for  its  subsequent  work- 
ing. 


PRACTICAL    MECHANICS.  315 

325.  Subterranean  works  of  research  may  be 
either  galleries  or  shafts.  The  former  are  passages 
cut  in  a  position  nearly  horizontal.  The  latter  are 
pits,  and  may  be  either  vertical  or  inclined.  Galle- 
ries may  be  either  directed  along  the  body  of  a  vein 
or  layer,  or  may  be  nearly  perpendicular  to  that  di- 
rection. Inclined  shafts  will  follow  the  inclination 
of  the  vein  downward,  while  those  which  are  vertical 
may  pass  through  the  rocks  in  which  it  lies. 

326.  When  the  rocks  which  overlie  the  mineral 
are  not  too  hard,  the  search  for  it  may  be  pursued 
by  boring.  The  instrument  usually  employed  is  no- 
thing more  than  a  large  auger,  the  shank  of  which 
is  composed  of  a  number  of  separate  pieces,  which 
are  joined  to  each  other  in  proportion  as  the  depth 
increases.  The  joint  is  formed  by  a  square  mortise, 
into  which  a  square  head  drops  ;  the  two  pieces  are 
then  keyed  together.  The  auger  is  provided  with  a 
number  of  cutters  of  different  forms,  each  adapted  to 
some  particular  description  of  soil. 

327.  Mines  are  sometimes  worked  open  to  the 
day  ;  this  is  usually  the  case  with  quarries  of  stone, 
and  always  with  alluvial  and  diluvial  ores  of  iron, 
with  clay  and  sand,  and  with  turf.  In  the  same  way 
are  worked  the  greater  part  of  the  lignites,  some 
beds  of  coal,  the  iron  of  Elba,  and  various  metallic 
minerals  in  Sweden  and  Norway.  In  this  country, 
the  great  beds  of  coal  which  are  found  at  Mauchunck 
have  hitherto  been  entirely  worked  in  this  manner, 
and  the  same  is  the  case  with  the  valuable  bed  of 
hematite  at  SaHsbury,  Conn. 

This  mode  of  working  can  only  be  profitably  per- 
formed when  the  mineral  lies  nearly  parallel  to  the 
surface,  and  the  whole  of  the  cover  can  be  easily  re- 
moved. 


316  PRACTICAL    MECHANICS. 

328.  Subterranean  works  alone  are  generally  ap- 
plicable to  the  greater  part  of  mineral  sites.  Such, 
however,  is  the  difference  in  the  manner  of  their  oc- 
currence, that  methods  of  very  different  character 
must  be  adopted,  corresponding  to  the  great  variety 
of  circumstances  in  which  valuable  minerals  occur. 
Minerals  may  occur  in  positions  which  can  be  class- 
ed under  five  distinct  heads. 

1.  Veins  or  beds  nearly  vertical,  or  having  an  in- 
clination  of  more  than  45°,  and  whose  average 
thickness  does  not  exceed  6  feet.  , 

2.  Beds  whose  thickness  is  not  more  than  6  feet, 
and  which  are  horizontal,  or  nearly  so. 

3.  Beds  of  great  thickness  and  sm.all  inclination. 

4.  Veins  or  beds  nearly  vertical,  and  of  great 
thickness. 

5.  Masses  whose  dimensions  are  considerable  in 
every  direction.  These  may  either  be  portions  of 
a  very  thick  bed,  or  a  space  the  whole  of  which 
must  be  extracted  in  consequence  of  the  number  of 
veins  which  intersect  it. 

329.  The  working  of  mines  includes  two  very 
distinct  classes  of  operations  :  those  which  are  pre- 
paratory, and  those  by  which  the  mineral  is  remo- 
ved. The  preparatory  works  consist :  of  galleries 
or  shafts,  by  which  the  miner  reaches  the  fittest  place 
for  beginning  to  extract  the  mineral ;  of  similar 
works,  by  which  the  site  of  the  mineral  is  reconnoi- 
tred ;  of  passages  of  either  description,  for  the  drain- 
age of  water,  for  the  circulation  of  air,  and  the 
transportation  of  the  minerals  after  they  are  separ- 
ated from  the  vein. 

When  a  vein  or  bed  is  situated  on  a  hill,  and  its 
direction  is  nearly  perpendicular  to  the  face  of  the 
height,  and  crops  out  at  its  surface,  a  gallery  should 
be  opened  in  the  outcrop  at  the  lowest  accessible 


PRACTICAL   MECHANICS.  317 

position.  This  will  serve  the  double  purpose  of 
draining  the  mine,  and  of  exploring  the  nature  and 
value  of  the  mineral,  in  the  direction  of  the  vein  it- 
self. Farther  researches  may  be  carried  on  by- 
pushing  galleries  or  inclined  shafts  upward  from  this 
main  gallery,  and  uniting  them  from  time  to  time  by 
cross  galleries.  In  all  works  of  this  latter  descrip- 
tion, it  is  important  that  they  be  laid  out  with  the 
greatest  regularity,  and  carried  on  in  lines  parallel 
and  perpendicular  to  each  other. 

When  the  vein  is  situated  in  a  hill,  and  is  nearly 
parallel  to  its  face,  a  gallery  is  driven  in  such  man- 
ner as  to  cut  the  vein  in  the  most  direct  manner, 
and  at  the  lowest  accessible  level.  A  cross  gallery 
is  then  cut  to  the  right  and  left  in  the  vein  itself,  and 
works  similar  to  those  last  mentioned  in  the  former 
case  are  pushed  upward  in  the  vein. 

When  a  bed  or  vein  is  much  inclined  to  the  hori- 
zon, a  vertical  shaft  should  be  sunk,  beginning  at 
some  distance  from  the  vein,  on  the  side  of  its  roof, 
and  be  carried  down  until  it  cuts  the  vein.  If,  how- 
ever, the  roof  is  of  such  a  nature  that  it  is  to  be 
feared  that  it  may  not  stand,  the  shaft  may  be  sunk 
on  the  side  of  the  floor,  and  a  horizontal  gallery 
driven  towards  the  vein,  until  it  is  cut  from  the  bot- 
tom of  the  shaft.  In  some  cases,  an  inclined  shaft 
may  be  sunk  in  the  vein  itself.  This  will  be  longer 
than  the  vertical  shaft,  will  be  less  solid,  and  more 
costly  in  its  structure ;  but  it  will  sometimes  cut 
through  ores  or  minerals  of  considerable  value.  It 
also  serves  to  explore  the  vein.  On  the  other  hand, 
the  part  of  the  vein  through  which  it  passes  caflnot 
be  touched  without  danger  ;  and  more  ore  will  thus 
be  lost  than  is  extracted  by  the  shaft  itself. 

If  the  vein  or  bed  be  nearly  horizontal,  and  lie 
25 


318         PRACTICAL  MECHANICS. 

deep  in  the  ground,  the  preparatory  work  must  also 
be  a  vertical  shaft. 

Were  it  not  for  the  expense,  it  would  be  better,  in 
both  these  cases,  to  sink  two  shafts,  in  order  to 
obtain  a  circulation  of  air.  These  must  be  united 
by  a  gallery  cut  in  the  vein  or  bed. 

When  two  veins  cross  each  other,  the  shaft  ought 
to  be  sunk  in  such  manner  as  to  cut  them  at  their 
common  intersection. 

330.  After  these  preparatory  works  are  finished 
and  those  of  extraction  are  commenced,  it  is  still 
necessary  to  carry  on  works  of  research,  in  order 
that  the  working  of  the  mine  may  not  be  suddenly 
checked.  These  works  of  research  consist  in  gal. 
leries  crossing  each  other  at  right  angles  when  the 
inclination  is  small,  or  in  inclined  pits  and  horizon- 
tal galleries  when  it  is  great.  They  are  to  be  cut 
in  the  vein  itself,  occupying  its  whole  thickness,  and 
are  extended  into  the  wall  and  roof,  or  even  into  the 
adjacent  rock,  when  the  vein  is  thin.  These  galler- 
ies or  shafts  must  be  not  more  than  from  80  to  150 
feet  apart.  In  some  cases  the  want  of  fresh  air  will 
make  it  necessary  to  cut  galleries  at  less  distances 
than  these  from  each  other. 

331.  The  spaces  into  which  the  vein  or  layer  is 
divided  are  then  to  be  taken  out.  This  is  done  by 
working  in  a  series  of  steps  when  the  inclination  is 
great,  or  by  galleries  crossing  each  other  at  smaller 
intervals  when  the  bed  or  vein  is  nearly  horizontal. 
In  the  latter  case,  no  more  of  the  vein  is  left  than 
pillars  of  sufficient  strength  to  support  the  roof. 
When  the  mineral  is  of  no  great  value,  these  pillars 
are  sometimes  lost  altogether ;  in  other  cases  they 
are  removed,  and  replaced  by  wooden  posts.  When 
the  whole  of  the  mineral  is  removed,  these  posts  are 


PRACTICAL   MECHANICS.  319 

sometimes  taken  out  and  the  roof  allowed  to  crush 
in.  This  operation  is  often  dangerous,  but  is  fre- 
quently absolutely  necessary,  to  maintain  the  value 
of  the  mine,  and  provide  for  its  being  worked  with 
ease. 

Of  all  mines,  whether  in  veins  or  beds,  those  which 
have  a  thickness  of  from  five  to  seven  feet  are  worked 
with  the  greatest  ease.  When  they  are  much  in- 
clined, the  whole  of  the  vein  or  bed  may  be  taken  out 
by  working  in  the  method  of  steps,  v/hich  has  been 
already  referred  to.  This  method  may  be  perform- 
ed either  by  beginning  at  the  top  or  at  the  bottom  of 
the  mass  to  be  excavated.  In  the  former  case,  a 
miner  begins  at  the  side  of  a  shaft,  about  six  feet 
below  a  former  working  or  the  upper  part  of  the 
vein,  and  excavates  a  space  in  the  vein  seven  feet 
in  height.  As  he  proceeds,  he  forms  a  platform  be- 
hind him  of  timber  and  plank,  on  which  he  piles  the 
refuse  matter  of  his  excavation.  As  soon  as  he  has 
proceeded  to  the  distance  of  seven  or  eight  feet  from 
the  shaft,  a  second  miner  begins  on  the  part  of  the 
vein  immediately  beneath  that  which  the  first  has  ex- 
tracted, and  proceeds  in  the  same  way.  In  the  mean- 
time, the  first  miner  continues  his  labours.  As  soon 
as  the  second  miner  has  proceeded  to  the  same  dis- 
tance of  from  seven  to  eight  feet  from  the  shaft,  a 
third  miner  is  set  to  work  beneath  him,  and  so  on, 
until  the  whole  space  on  the  side  of  the  shaft  between 
two  horizontal  galleries  is  occupied. 

In  the  second  case,  a  horizontal  gallery,  well  sup- 
ported with  timber,  is  carried  forward  in  the  vein 
from  the  bottom  of  a  shaft.  The  first  miner  begins 
to  work  on  the  roof  of  this  gallery,  and  throws  the 
rubbish  on  the  timber  which  covers  it.  This  forms 
an  inclined  plane,  on  which,  after  he  has  proceeded 
seven  or  eight  feet,  a  second  miner  may  be  set  tc 


320         PRACTICAL  MECHANICS. 

work  behind  him,  and  so  on  until  the  whole  space  is 
occupied.  This  method  is  applicable  in  veins  which 
are  as  thin  as  eighteen  inches. 

Veins  of  considerable  inclination  and  great  thick- 
ness are  pierced  by  means  of  horizontal  galleries ; 
and  it  is  necessary,  unless  when  timber  is  very  abun- 
dant, to  leave  floors  and  partitions  of  the  mineral  be- 
tween the  several  galleries.  The  mineral  thus  left 
is  wholly  lost. 

332.  In  very  thick  beds,  the  work  is  conducted  by 
dividing  it  into  stages  of  about  six  feet  in  thickness. 
For  this  purpose  advantage  is  taken,  if  possible,  of 
natural  partings,  which  often  exist  in  the  bed.  The 
working  is  begun  in  the  lower  stage,  and  is  carried 
on  by  a  series  of  galleries  and  rooms  crossing  each 
other  at  right  angles,  leaving  pillars  in  their  inter- 
sections. In  laying  out  the  workings  of  the  second 
and  upper  stages,  the  greatest  care  must  be  taken 
that  their  several  galleries  and  rooms  lie  immediately 
above  those  of  the  lower  stages,  so  that  the  pillars 
may  in  fact  be  continuous  from  the  floor  to  the  roof 
of  the  bed. 

If  there  be  any  quantity  of  refuse  matter,  it  must 
be  used  to  fill  up  the  galleries  of  the  lower  stages, 
and  in  this  case  new  galleries  may  be  cut  in  the  pil. 
lars,  by  which  an  additional  quantity  of  mineral  can 
be  taken  out.  The  upper  stage,  if  the  refuse  matter 
be  sufficient  to  fill  up  all  the  lower  galleries,  may  be 
treated  like  a  thin  bed,  and  the  whole  of  the  mineral 
extracted. 

In  very  thin  beds  or  flat  veins,  the  main  galleries 
by  which  the  country  is  explored  must  be  cut  into 
the  roof,  and  made  high  enough  to  permit  the  free 
motion  of  the  workmen,  and  of  the  vehicles  by  which 
the  ore  is  removed.  The  intervening  portions  of 
the  layer  are  worked  by  men  lying  on  their  sides. 


PRACTICAL    MECHANICS.  321 

When  masses  of  considerable  thickness  exist,  at  no 
great  depth,  shafts  may  be  sunk  until  they  cut  the 
mineral ;  these  are  enlarged  in  every  direction  into 
the  form  of  a  cone  or  conoid,  until  there  begins  to  be 
danger  that  the  rock  will  no  longer  support  itself. 
The  shaft  is  then  abandoned  and  a  new  one  com- 
menced. 

The  parts  of  the  mine  nearest  to  the  point  where 
the  entrance  gallery  or  main  shaft  enters  it,  ought 
aot  to  be  worked  until  all  those  which  are  more  dis- 
:ant  have  been  exhausted. 

333.  The  dangers  to  which  miners  are  exposed 
are  the  falling  of  the  rocks,  which  are  always  di- 
vided by  joints  or  fissures  ;  the  crushing  of  loose 
earth  or  decomposed  rock,  and  particularly  of  quick- 
sand ;  accumulations  of  water,  and  collections  of  foul 
air. 

334.  The  galleries  and  shafts  may  be  cut  in  strong 
and  firm  rock  ;  in  this  case  they  will  need  little  or 
no  support.  When  this  is  not  the  case,  shafts  re- 
quire to  be  surrounded  by  a  kerb.  This  may  be 
composed  of  timber  or  of  masonry.  In  the  former 
case  the  shaft  must  be  rectangular,  in  the  latter 
case  it  is  circular  or  elliptical. 

Galleries  are  sustained,  when  necessary,  by  a  se- 
ries of  frames,  each  composed  of  three  pieces  of  tim- 
ber. These  frames  are  usually  about  three  feet 
apart,  and  if  the  ground  on  the  sides  be  bad,  the 
spaces  between  them  are  filled  up  with  short  bars  of 
wood.  The  roof  of  the  gallery  is  formed  in  the  same 
way. 

The  spaces  between  the  galleries  whence  ore  is 
"extracted  are  supported  by  posts,  unless  the  working 
itself  furnishes  a  sufl[icient  quantity  of  rubbish  to  form 
pillars  wherewith  to  hold  up  the  roof. 


322  PRACTICAL    MECHANICS. 

335.  Whenever  it  is  practicable,  mines  should  be 
drained  of  water  by  means  of  a  horizontal  gallery. 
That  which  has  been  spoken  of  among  the  works  of 
preparation  will  often  answer  the  purpose.  In  this 
case  it  is  to  be  divided  into  two  parts  by  a  horizon- 
tal floor,  beneath  which  the  water  may  run,  and  on 
which  the  mineral  may  be  carried  out.  The  great- 
est slope  of  a  gallery  of  drainage  ought  not  to  ex. 
ceed  ^i^th. 

When  the  part  of  the  vein  which  lies  above  the 
gallery  is  exhausted,  it  becomes  necessary  to  work 
the  mine  beneath  that  level.  In  this  case  artificial 
means  of  drainage  must  be  employed,  and  it  was  to 
m.eet  an  instance  of  this  sort  that  the  machine  of 
Schemnitz,  described  in  §  209,  was  invented.  On 
the  Continent  of  Europe  and  in  Spanish  America, 
the  law  provides  for  a  division  of  the  expense  of  a 
gallery  of  drainage  among  those  who  are  benefited 
by  it.  Under  the  protection  of  this  law,  a  gallery 
of  several  miles  in  length  has  been  constructed  in 
Saxony.  This  gallery  passes  at  a  depth  of  900  feet 
beneath  a  village,  and  took  23  years  to  construct. 

In  sinking  shafts,  and  in  drainage  of  no  great 
depth,  horse  power  may  be  used.  The  best  arrange- 
ment for  the  use  of  this  kind  of  force  is  called  the 
whim.  It  is  a  capstan,  to  the  bar  of  which  two 
horses  are  harnessed.  A  rope  is  wound  around  the 
barrel  of  this  windlass,  and  has  a  bucket  at  each 
end,  so  that  as  one  bucket  is  drawn  up,  the  other  de- 
scends. The  horse-path  is  so  wide  that  the  horses 
can  be  turned  around  for  the  purpose  of  reversing 
their  motion.  The  best  buckets  for  this  purpose 
are  made  of  two  hides  sewn  or  riveted  together. 
The  tails  form  a  pipe  or  passage  through  which  the 
water  enters  when  the  bucket  descends  into  the 
well  at  the  bottom  of  the  shaft.     This  pipe  is  then 


PRACTICAL  MECHANICS.         323 

lifted  and  hooked  to  the  top  of  the  bucket.  When 
it  reaches  the  top  of  the  shaft,  the  pipe  is  unhooked, 
and  the  water  is  discharged  through  it. 

These  buckets  were  originally  employed  in  Mex- 
ico, and  were  made  of  raw  hides  ;  in  those  which 
have  been  used  in  this  country,  the  hides  are  tanned, 
by  which  they  are  rendered  much  more  durable. 

The  most  perfect  of  all  modes  of  drainage  is 
the  forcing  pump  moved  by  a  steam-engine.  When 
the  depth  exceeds  130  feet,  two  stages  of  pumps  are 
necessary,  and  an  additional  stage  for  every  130  feet 
in  addition.  In  sinking  a  shaft,  the  pump  may  be 
used  almost  from  the  beginning  of  the  work.  It 
is,  for  this  purpose,  fastened  to  a  wooden  frame,  the 
lower  end  of  which  rests  on  the  bottom  of  the  exca- 
vation. 

The  single-acting  engine,  working  expansively,  has 
been  preferred  for  working  the  pumps  of  mines  ;  but 
double-acting  condensing,  and  high-pressure  en- 
gines have  also  been  successfully  employed.  What- 
ever be  the  mode  in  which  water  is  raised  in  a  ver- 
tical shaft,  it  is  necessary  that  the  excavation  be 
continued  for  some  feet  below  the  workings,  in  order 
to  form  a  well  for  the  collection  of  the  water  of  the 
mine. 

336.  The  air  of  mines  is  rendered  foul  by  the 
breath  of  the  workmen,  the  combustion  of  their 
lights,  and  the  decomposition  of  the  wood  which  is 
employed  in  supporting  the  galleries,  &c.  Carbonic 
acid  in  addition  is  often  evolved  by  the  waters,  and 
carburetted  and  sulphuretted  hydrogen  are  given 
out  from  beds  of  coal.  Mines  of  metals  often  give 
out  arsenical  vapours,  and  those  of  mercury  the  va- 
pour  of  that  metal. 

Not  only  are  these  gases  and  vapours  injurious, 
and  even  destructive  when  breathed,  but  the  hydro- 


324  PRACTICAL    MECHANICS. 

gen  forms  with  atmospheric  air  an  explosive  com- 
pound, which,  if  entered  by  a  light,  produces  the 
most  disastrous  effects. 

For  these  reasons,  it  is  of  the  utmost  importance 
that  mines  be  well  ventilated.  This  should  be  done 
wherever  it  is  possible  by  natural  currents  of  air, 
but  it  is  often  necessary  to  resort  to  artificial  means. 

The  principle  of  natural  ventilation  depends  upon 
the  fact  that  there  is  generally  a  difference  between 
the  temperature  of  mines  and  that  of  the  external 
air.  In  winter  the  mines  are  always  warmer  than 
the  atmosphere,  and  in  summer  they  are  often  cold- 
er. So  long  as  the  difference  in  temperature  is  con- 
siderable, it  is  only  necessary  that  the  mine  should 
have  two  openings  at  different  levels,  and  the  exter- 
nal air  will  enter  by  the  one  and  escape  by  the  oth- 
er. The  works  of  the  mine  must  be  formed,  by 
means  of  partitions  of  plank,  crossing  and  closing 
galleries  into  a  single  series  of  passages,  by  which 
the  air  will  circulate  backward  and  forward  from 
the  time  it  enters  the  mine  until  it  reaches  the  place 
of  discharge.  In  these  partitions  doors  must  often 
be  placed,  in  order  to  permit  the  passage  of  the 
workmen  and  of  the  matter  extracted  from  the  mine. 
It  will  be  absolutely  necessary  to  keep  these  doors 
open  for  the  shortest  possible  time,  for  they  admit  a 
more  direct  passage  for  the  air,  and  thus  all  the 
ventilation  of  the  inore  distant  galleries  will  be  in- 
tercepted. 

Even  a  single  opening,  whether  it  be  a  gallery  or 
a  shafl,  may  serve  to  ventilate  a  mine.  In  the  case 
of  a  gallery,  a  short  shaft  may  be  sunk  at  no  great 
distance  from  its  entrance,  reaching  from  the  gallery 
to  the  surface  of  the  ground.  The  gallery  is  then 
to  be  divided  into  two  parts  by  a  horizontal  parti- 
tion ;  the  upper  part  communicates  with  the  shafl, 


PRACTICAL    MECHANICS.  325 

and  the  two  parts  communicate  only  at  the  farthest 
extremity  of  the  gallery.  The  water  which  runs 
in  the  gallery  will  aid  in  causing  a  current  of  air. 

When  there  is  no  more  than  one  shaft,  it  must  be 
divided  into  two  parts  by  a  vertical  partition  of 
plank,  carried  down  to  the  very  bottom,  or,  at  least, 
beneath  the  surface  of  the  water  in  the  welL  The 
galleries  on  each  side  of  the  shaft  communicate  with 
one  of  its  two  divisions,  and  the  same  arrangement 
of  partitions  is  made  as  in  the  case  of  two  galleries, 
so  as  to  form  one  continuous  passage  to  and  fro,  by 
which  the  air  that  descends  through  one  of  the  com- 
partments of  the  shaft  may  circulate  throughout 
the  whole  mine  before  it  reaches  the  other.  The 
importance  of  making  the  partitions  tight,  and  of 
keeping  any  doors  which  may  be  left  in  them  as 
much  closed  as  possible,  is  even  more  important  in 
this  case  than  in  that  of  galleries. 

It  is  to  the  opening  of  a  door  in  a  mine  arranged 
for  ventilation  in  this  way,  that  the  fatal  explosion  at 
Blackheath,  in  Virginia,  is  to  be  attributed.  By 
this  explosion  60  workmen  lost  their  lives. 

The  mere  agitation  produced  by  the  working  of 
the  pumps  in  one  of  the  compartments  of  the  shaft 
is  often  sufficient  to  change  the  temperature  of  the 
air  in  it  and  cause  a  current,  but  it  is  more  safe  to 
place  a  chimney  in  communication  with  the  smaller 
compartment,  by  which  the  advantage  of  a  differ- 
ence of  level  in  the  openings  will  be  attained. 

A  more  secure  ventilation  will  be  effected  in  this 
case  by  the  artificial  means  of  heat.  A  fire  may 
be  lighted  in  one  of  the  compartments  of  the  shaft 
at  some  distance  from  the  bottom,  and  the  chimney 
in  this  case  will  be  unnecessary.  When  there  is  a 
chimney,  a  fire  may  be  built  in  a  furnace  placed  be- 
tween it  and  one  of  the  compartments  of  the  shaft;. 


326  PRACTICAL    MECHANICS. 

This  furnace  must  have  no  other  openings  than  those 
by  which  it  communicates  with  the  shaft  and  chim- 
ney, except  the  door  for  feeding  the  fire,  and  this 
should  be  only  opened  when  the  fuel  is  thrown  in. 

Until  the  mine  is  sufficiently  opened  to  permit  its 
being  arranged  in  a  system  of  passages  for  ventila- 
tion, and  when  the  difference  in  the  temperature  of 
the  internal  and  external  air  is  not  sufficient,  other 
artificial  means  may  be  resorted  to.  Thus  fresh  air 
may  be  forced  in  by  bellows  or  blowing  machines  of 
various  descriptions.  In  this  way,  however,  the  fresh 
air  is  merely  mixed  with  the  foul,  and  the  ventilation 
is  never  complete.  It  is  otherwise  when  the  foul  air 
is  pumped  from  the  mine  by  similar  machines  work- 
ing in  an  opposite  direction.  The  simplest  machine 
which  has  ever  been  constructed  for  this  purpose  is 
formed  of  one  tub  inverted  within  another,  filled 
with  water.  A  pipe  passes  through  the  bottom  of 
the  latter,  rising  above  the  surface  of  the  water  it  con- 
tains, and  is  extended  to  the  most  distant  point  of 
the  mine,  or  to  the  discharging  end  of  the  series  of 
passages.  The  first  tub  has  upon  its  bottom,  which, 
as  it  is  inverted,  is  uppermost,  a  valve  opening  up- 
ward. The  machine  is  worked  by  attaching  the 
first  tub  to  the  pumping  engine,  by  which  it  is  al- 
ternately  raised  and  lowered. 

In  coal-mines  the  danger  of  explosion  might  be 
avoided  altogether  by  the  use  of  the  safety-lamp  of 
Davy,  were  it  not  that  the  wire  gauze  with  which  it 
is  covered  is  liable  to  be  torn,  and  it  is  hardly  possi- 
ble to  compel  workmen  to  pay  sufficient  attention  to 
keep  them  in  proper  order.  When  there  is  even  a 
small  rent  in  them,  they  are  as  dangerous  as  an  un- 
covered light,  and  with  this  addition,  that  confidence 
is  reposed  in  them. 


PRACTICAL  MECHANICS.  327 


337.  We  jfiave  thus  completed  an  attempt  to  give, 
in  a  condensed  form,  a  view  of  many  of  the  more 
important  applications  of  the  science  of  mechanics 
to  industry.  One  of  great  moment  has  been  pur- 
posely omitted,  namely,  the  principles  of  architecture 
and  the  practice  of  building.  These  are  of  sufficient 
moment  to  require  a  treatise  to  themselves,  and  ma- 
terials for  such  a  work  are  in  preparation. 

The  view  we  have  given  of  the  application  of 
prime  movers,  and  particularly  of  the  steam-engine, 
is  calculated  to  give  us  an  exalted  opinion  of  the 
powers  of  the  human  mind,  in  its  influence  not  only 
over  inert  matter,  but  over  the  elements  themselves. 
Still,  however  lofty  may  be  the  estimate  we  thus  form 
of  the  achievements  of  human  genius,  it  will  be  seen 
that  all  which  the  utmost  exertion  of  skill  or  talent 
has  been  able  to  effect,  or,  indeed,  can  ever  accom- 
plish, is  to  bring  into  action  powers  and  agents,  pro- 
vided not  merely  for  our  own  use,  but  for  the  fulfil- 
ment of  the  most  important  purposes  in  the  creation. 
We  not  only  bridle  the  wild  steed,  and  compel  him 
to  bear  burdens  or  draw  loads,  but  we  intercept  the 
waters  in  their  return  course  to  the  ocean,  catch 
upon  sails  the  whirling  currents  of  the  atmosphere, 
confine  the  imponderable  element  of  heat,  and  com- 
pel it  to  expand  an  inert  and  inactive  fluid  into  va- 
pour. Mighty  as  are  the  effects  that  are  thus  pro. 
duced,  they  are  no  more  than  applications  of  forces 
to  whose  source  and  origin  we  cannot  approach  ;  and 
man  in  the  exertion  of  his  highest  powers  over  mat- 
ter,  is  only  rendered  the  more  sensible  of  his  depend- 
ance  upon  the  Creator  not  only  of  his  own  frame, 
but  of  the  natural  agents,  by  the  use  of  which  he  is 
enabled  to  accomplish  so  many  important  objects 

THE   END. 


-r.-v. 


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B  liRPARY     I1NIVPP.<;ITY    OF    T Al  IPOPKIIA      HAVIC 


3  1175  00485  21 1( 


Call  Number: 


76U313 


Renwick,  J. 

Applications  of  the 
science  of  mechanics  to 


TJIU6 
Rl^2 


N?  764313 

Renwick,   J. 

Applications  of  the 
science  of  mechctnics  to 
practical  p\irposes» 


TJIU6 
Bh2 


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